Trailer Brake Lights Not Working On a Trailer Towed by a 2003 Saab 9-3

Question:

I cant get my brake light to work. what should I try doing?

asked by: Larry

Expert Reply:

There are a few possibilities as to why the trailer brake lights are not working. First, you will need to make sure that you have the wiring from the harness connected to the proper wires on the 2003 Saab 9-3. The # C59496 harness can be wired in different ways depending on how the vehicle tail lights are wired.

To determine if the vehicle is the problem:
You will need to determine if the vehicle has a 2 wire or 3 wire system. A 2 wire system will have the same bulb for stop and turn signals. If the vehicle has one combined circuit and one just for stop lights as well you will still use the combined circuit for the harness. If you identify your vehicle as a 2-wire system then you will ground the red wire with the white wire.

A 3 wire system usually has amber turn signals and separate bulbs for stop and turn signals. You will need to use the included circuit tester to test the left side wiring to determine which wire carries the left turn signal and attach the yellow wire from the harness to it. Then find the running light wire and attach the harness brown wire. And then find the stop light only wire and attach it to the harness red wire.

The green wire will get routed to the passenger side and connect to the right turn only wire on the vehicle.

If you have everything wired correctly you will want to check and make sure that there are no stop light or CHMSL fuses that are blown. Use the owners manual or the legend on the power distribution box lid to find the locations of these fuses, if present, and replace them as needed.

To determine if the trailer or wiring harness is the problem:
If everything is wired correctly and the fuses are not the issue then the problem is most likely on the trailer. Disconnect the trailer and use the circuit tester to test the 4-Way end on the vehicle. If the brake lights are working there that verifies an issue on the trailer. If not then you will need to test the converter.

Use the circuit tester to test the wires going into the converter box. If the signals are correct going in, test the wires coming out. If they are not correct coming out then there is an issue with the converter and it will need to be replaced. If the signals are not correct coming in then there is a problem with the vehicle wiring.

If everything checks out on the vehicle end you can move on to the trailer. Make sure the 4-Way connector is clean and free of corrosion. Then move on to checking the grounds. The main trailer connector ground and the tail lights will all need to be grounded to clean and corrosion free bare metal surfaces. If you have a tilt trailer or a folding tongue then you will want to run a jumper ground from the lights to the main trailer connector ground.