Choosing a Tow Bar Wiring Harness

Your vehicle is one of your most valuable possessions, so wiring up a tow bar to pull it behind your trailer can seem pretty intimidating. But the good news is that it doesn't have to be. Every vehicle has custom-fit wiring options, so choosing the right wiring kit is just a matter of choosing between these custom options.

So what are your options? It depends on your vehicle, but in general, you can splice into your car's wiring, drill into your tail light housings, or run wiring beneath your car. Depending on your specific vehicle, you may also have to splice in additional accessories for your supplemental braking system.

Our fitguide can help you narrow down your choices and let you know which additional components (if any) your vehicle needs.

How to Choose a Tail Light Wiring Kit

When you begin the wiring process, ask yourself the following two questions:

1. Does your towed vehicle have separate or combined lighting?

Combined Lighting System - one bulb inside each tail light housing illuminates for both the brake and turn signal functions of your vehicle. This is sometimes referred to as a "2-wire system".

Separate Lighting System - In each tail light housing, one bulb illuminates for the brake signal and a different bulb illuminates for the turn signal. Typically, vehicles with separate lighting (or a 3-wire system) have red brake lights and amber turn signals.

2. Do the brake lights on your towed vehicle illuminate when the engine is off?

Test to see if your car's brake lights activate when the ignition is in the "tow" position.

Choose the scenario below that applies to you, based on the above questions.

In this case, you must decide if you want your towed car to maintain separate brake and turn signal functions while it's being towed, or if you want to wire it as a combined system.

If you want to keep the signals separate:

Wire your towed vehicle using either a 6-diode kit or a block-style diode that works for 3-wire systems. If your RV also has a separate lighting system and you have a 6-way or 7-way vehicle-side connector on your towed car, one of these kits is all that you need. If your RV operates on a combined system, or if it is wired only with a 4-way connector at the rear, you need to install a 2-to-3 tail light converter on your towed vehicle.

Or, use a custom wiring harness to maintain your towed car's wiring, ensuring that the signals travel along their separate wires. This kit will, however, cause your stop and turn lights to function in tandem. In this way, a wiring system makes your towed vehicle's wiring operate, more or less, in a combined fashion.

If you want your towed vehicle to adopt a combined lighting system (only while being towed):

You can use a 4-diode kit, a block-style diode or a custom wiring harness. A brake light relay is also required for use with a supplemental braking system.

Use a 4-diode kit, block-style diode, or custom wiring harness. A brake light relay is also required for use with a supplemental braking system.

Refer back to number 1 to choose a tail light wiring kit. In addition to the wiring kit, you need to install a stop light switch kit.

Refer back to number 2 to choose a tail light wiring kit. In addition to the wiring kit, you need to install a stop light switch kit.

The following table gives an easy-to-understand breakdown of the wiring options available for your particular application. Consult this table to determine what electrical items you need to wire your towed vehicle based on the given criteria.

Towed Vehicle: Combined Lighting

RV: Combined Lighting or 4-Way Trailer Connector

Brake Lights Illuminate

OPT 1: Wiring Harness + Brake Light Relay

OPT 2: 4-Diode Kit + Brake Light Relay

OPT 3: Block Diode Kit + Brake Light Relay

Brake Lights DO NOT Illuminate

OPT 1: Wiring Harness + Stop Light Switch

OPT 2: 4-Diode Kit + Stop Light Switch

OPT 3: Block Diode Kit + Stop Light Switch

Brake Lights Illuminate

OPT 1: Wiring Harness + Brake Light Relay

OPT 2: 4-Diode Kit + Brake Light Relay + Brite-Lite Converter

OPT 3: Block Diode Kit + Brake Light Relay + Brite-Lite Converter

Brake Lights DO NOT Illuminate

OPT 1: Wiring Harness + Stop Light Switch

OPT 2: 4-Diode Kit + Stop Light Switch + Brite-Lite Converter

OPT 3: Block Diode Kit + Stop Light Switch + Brite-Lite Converter

Brake Lights Illuminate

OPT 1: Wiring Harness

OPT 2: 6-Diode Kit w/ 2-to-3 Converter

OPT 3: Block Diode Kit + 2-to-3 Converter

Brake Lights DO NOT Illuminate

OPT 1: Wiring Harness + Stop Light Switch

OPT 2: 6-Diode Kit w/ 2-to-3 Converter + Stop Light Switch

OPT 3: Block Diode Kit + Stop Light Switch + 2-to-3 Converter

Brake Lights Illuminate

OPT 1: Wiring Harness

OPT 2: 6-Diode Kit

OPT 3: Block Diode Kit

Brake Lights DO NOT Illuminate

OPT 1: Wiring Harness + Stop Light Switch

OPT 2: 6-Diode Kit + Stop Light Switch

OPT 3: Block Diode Kit + Stop Light Switch

Types of Tail Light Wiring Kits

The first item you need to choose when wiring your towed vehicle is a tail light wiring kit. A kit of this nature is required on all vehicles that are being towed. To legally flat tow your vehicle, you must ensure that it is properly wired and connected to your RV. Your towed vehicle's signal lights must act in accordance with the brake and turn signal lights on your RV. There are a few ways that you can achieve this:

Removable Tail Light Kits

The least invasive method of wiring your towed vehicle, these lights mount on your car with magnets or in your trunk with special tabs. The wiring runs beneath your vehicle and up to the front so you can connect to your RV. This type of wiring bypasses your towed vehicle's wiring entirely.

Bulb and Socket Kits

These lights feature a minimally invasive design. Bulbs mount in your towed car's tail light housings (a hole must be drilled in each tail light housing).

The wiring runs beneath your vehicle and up to the front so you can connect to your RV. These types of kits bypass your towed vehicle's wiring entirely. They require a one-time setup and activate automatically when you hook your towed car up to your RV.

Plug-In Vehicle Wiring

Plug-in kits tie into your towed vehicle's wiring. These custom-designed harnesses plug into your existing tail light wiring harness so that the towed car's tail lights emit signals sent by the RV. No cutting or splicing is required. Built-in diodes prevent electrical feedback so that your towed car's electronics are not damaged. These harnesses require a one-time setup and activate automatically when you hook your towed car up to your RV.

Diode Kits

Diode kits are the most invasive wiring option, since they hardwire into your towed vehicle.

Diodes are spliced into the vehicle's wiring so the towed car's tail lights emit signals sent by the RV. They prevent electrical feedback so the towed car's electronics are not damaged. They require a one-time setup and activate automatically when you hook your towed car up to your RV.

A 4-diode kit is required for vehicles with combined lighting systems. A 6-diode kit is required for vehicles with separate lighting systems. For more info, check out our help articles on installing diodes on a dingy with a combined or separate system.

You can also read more about the benefits of different wiring kit types here.

Brake Light Relay

The purpose of a brake light relay kit is to prevent your towed vehicle's brake signal from overriding the turn signal that comes from your RV. When your towed vehicle is properly wired and connected to your RV, its indicator lights will illuminate as you engage the turn signals in your RV. However, if your towed vehicle has a combined lighting system (or if you wire your towed vehicle to have a combined system while towing), the turn signal will be overridden by your towed car's brake signal as soon as your supplemental braking system presses the towed car's brake pedal. As a result, drivers behind you know only that you're slowing down and not that you are turning.

Installing a brake light relay in your towed vehicle fixes this problem by ensuring that your car's brake signal does not override the RV's turn signal. Therefore your towed car's lights will function in accordance with your RV at all times.

A brake light relay is required in your tow bar wiring setup if all of the following are true:

You are using a supplemental braking system
Your towed vehicle's brake lights illuminate when its engine is of
You are wiring your towed car as a combined lighting system (for towing) with one of the following:

Is a Relay Required?

To determine if a brake light relay is needed to properly wire your towed vehicle, answer the following questions:

1) Are you using a supplemental braking system that depresses the brake pedal in your towed car?

No - You do not need a brake light relay.

Yes - You may need a brake light relay. Continue on to the next question.

2) Do the brake lights on your towed car illuminate while the engine is off?

No - You do not need a brake light relay.

Yes - You may need a brake light relay. Continue on to the next question.

3) Do your towed car's brake lights act as both the brake signals and the turn signals?

No - You may need a brake light relay. Continue on to the next question.

Yes - Your towed vehicle has a combined lighting system. You will need a brake light relay unless you use a TowDaddy custom wiring kit, which has a relay built in.

4) Does your towed car have separate brake lights and turn signals?

No - Your towed vehicle has a combined lighting system. You will need a brake light relay unless you use a TowDaddy custom wiring kit, which has a relay built in.

Yes - Your towed vehicle has a separate lighting system. You may need a brake light relay. Continue on to the next question.

5) Are you wiring your vehicle with a 6-diode kit?

No - You will need a brake light relay to ensure proper activation of your towed car's signal lights.

Yes - You do not need a brake light relay.

Stop Light Switch Kit

A stop light switch kit is an accessory that, like a brake light relay, is designed to work with towing setups that use supplemental braking systems. Most braking systems include a monitor that mounts in your RV's cab to keep you alerted to your towed vehicle's braking activity. However, most of these monitors only alert you if the braking system has been activated, not if the towed car's brakes are actually being depressed.

If your braking system is not properly positioned, you may get a false reading on the monitor. You may think that your towed vehicle's brakes are being activated when in reality the braking system is missing the brake pedal entirely or simply not applying the necessary amount of force to slow the vehicle. This often happens with portable systems that require repositioning for every use. If improperly positioned, these systems can "crawl" up your driver's seat, negatively affecting contact with the brake pedal.

It is also possible that your braking system is positioned so that it is constantly applying pressure to your towed car's brake pedal whether the system is activated or not. If this is the case, the system will apply excessive pressure when it activates, which can create extreme wear on your brakes and cause damage to your tires and various other braking components.

With a stop light switch kit, you always know when the brake in your towed vehicle is being depressed so that you can avoid costly damage. This kit adds another brake light switch to your towed car and powers it so that the monitor in your RV is activated when the brake pedal is depressed and not simply when the braking system is activated. This is done by connecting the switch directly to a power source - your vehicle's battery or fuse block - and also connecting it to the motorhome monitor output wire.

Is a Stop Light Switch Kit Required?

If you have a supplemental braking system and the brake lights on your towed vehicle do not illuminate when its engine is off - or when the ignition is in the "tow" position - you need a stop light switch kit.

If a stop light switch kit is not required for your towing setup, but you want to know whether or not your towed vehicle's brake pedal is being depressed, then you can still install a stop light switch kit. It is not, however, necessary.

Fuse Bypass Systems

Some vehicles can only be towed if you get under the hood and pull a fuse. If you don't remove this fuse prior to towing, there's a good chance that your car's battery will be drained by the time you reach your destination.

TowDaddy's AutoFUSE and Blue Ox's Fuse ByPass are both designed to make it easy to prep your vehicle for towing. Simply remove the fuse indicated in your car's manual and replace it with the bypass. Both of these systems are designed to hold the removed fuse and maintain its functionality. So, once the initial installation is complete, you don't have to remove or replace your fuse again.

The AutoFUSE activates automatically, bypassing the fuse when your car is hooked up to your RV (and the RV's running lights are on). And, when you are disconnected from the RV and driving your car, the fuse functions exactly as it would normally. In addition, the AutoFuse maintains charge to your towed car's battery so that you can continue to power accessories - like a supplemental braking system - while you tow, without draining your battery.

The Fuse ByPass also maintains your fuse's functionality when you're driving your car. But instead of automatically bypassing the fuse when your car is hooked up to your RV, the ByPass requires that you pop your car's hood and flip the integrated switch to activate or deactivate the fuse.

Is a Fuse Bypass System Required?

A fuse bypass is never required. But if you have to remove a fuse to tow your car, a bypass system makes the whole ordeal easier. To determine if you need to remove a fuse before towing, consult your vehicle's owner's manual.

Automatic Battery Disconnect

It is required that the battery be disconnected on certain vehicles before they can be towed. Because disconnecting your car's battery can be a chore, and a potentially dangerous one at that, Roadmaster developed an automatic battery disconnect. With this device, you only need to disconnect your battery once. After the initial installation, the disconnect will operate automatically, bypassing the battery whenever your car is hooked up to your RV.

This system also maintains your battery's charge by pulling power from your RV. So even if your vehicle doesn't need to have the battery disconnected to be towed, you can use the disconnect to prevent dead battery. This may be especially beneficial if you have a supplemental braking system that connects to your towed vehicle's 12-volt outlet to pull power from your vehicle's battery.

Is an Automatic Battery Disconnect Required?

A battery disconnect is never required. But if you have to disconnect your vehicle's battery before towing, this bypass system will make doing so safer and easier. To determine if you need to disconnect your vehicle's battery before towing, consult your vehicle's owner's manual.

Tail Light Converters

A tail light converter ensures that the wiring system on your RV matches the wiring on your towed vehicle. There are two types of converters:

2-to-3

Use this type of tail light converter if your RV has combined lighting (2-wire system) and your towed vehicle has separate lighting (3-wire system). By installing this converter in conjunction with the appropriate tail light wiring kit, you can maintain the separate lighting of your towed vehicle while you flat tow.

3-to-2

Use this type of tail light converter if your RV has separate lighting (3-wire system) and your towed vehicle has combined lighting (2-wire system). If your RV is wired with a 4-way trailer connector at the rear, it most likely has a 3-to-2-wire converter already built in.

Note: These tail light converters are not interchangeable. They cannot be installed backwards, with the input directed towards the output side.

Is a Tail Light Converter Required?

A tail light converter is required if the wiring system on your RV does not match the wiring system of your towed vehicle.

Choose 118158 or C56196 (2-to-3-wire converters) if your RV has a combined system and your towed vehicle has a separate system.
Choose RM-732 (3-to-2-wire converter) if your RV has a separate system and your towed vehicle has a combined system. (And your RV or tow vehicle does not have a 4-way trailer connector.)

2-to-3 Converter

3-to-2 Converter

Troubleshooting Tow Bar Wiring

If you're still not sure what kind of tow bar wiring accessories you might need, check out the table below. If the problems shown are ones that you've been experiencing and you don't currently have any of these items installed, you should look into these parts for possible solutions.

PROBLEM: Brake signal from towed car overrides turn signal from RV

POSSIBLE SOLUTION: Brake Light Relay

PROBLEM: Braking system's in-cab monitor indicates when system is activated, not when towed car's brake pedal is depressed

POSSIBLE SOLUTION: Stop Light Switch Kit

PROBLEM: Tires or brake components on towed car have been damaged due to overbraking

POSSIBLE SOLUTION: Stop Light Switch Kit

PROBLEM: Towed car's battery is dead by the time you reach your destination

POSSIBLE SOLUTION: TowDaddy AutoFUSE, Blue Ox Fuse Bypass, or Roadmaster Automatic Battery Disconnect

PROBLEM: You have a towed car with combined lighting, but an RV with separate lighting

POSSIBLE SOLUTION: Tail Light Converter

PROBLEM: You have a towed car with separate lighting, but an RV with combined lighting

POSSIBLE SOLUTION: Tail Light Converter

Last Updated: 1/15/22

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