Why do Trailer Wiring Harnesses Have Separate 12 Volt Power Wire on Newer Vehicles

Question:

On my 1986 F150, 2008 Kia Sorento, and 2013 Toyota Highlander, I did not have to run a hot wire to the battery. I did, however have to on my 2011 Kia Sorento and 2016 Sienna. WHYYYY in this day and age are these not plug and play wiring??! Such a painstaking process. Is it the vehicle manufacturer or the wiring manufacturer that is failing here? Had I ordered the stock tow package, there would not be a separate wire running to the battery. I don’t understand why this is.

asked by: Shawn

Expert Reply:

The separate power wire for trailer wiring harnesses is becoming more and more prevalent these days because vehicle manufacturers are designing their tail light circuits to use less power. More and more vehicles are coming out with LED tail lights or advanced circuitry like pulse width modulation. This means that the factory tail light wires on the vehicles are not designed to handle the extra load of the trailer lights.

Trailer wiring harness manufacturers like Tekonsha, Curt and Hopkins, test each vehicle to determine if their tail light circuits can handle the extra load because they know people would rather have a complete plug and play harness versus having to run and splice wires. Unfortunately, as vehicles become more and more advanced, the towing world is always playing catch-up.

To explain your point about the factory tow package, the reason they don't require a wire similar to aftermarket tow packages is because vehicles with factory tow packages have their own tow package circuit that is separate from the vehicle's tail light circuits. The factory tow package is wired in a way to draw power directly from the vehicle's battery to power the trailer lights, just like the aftermarket harnesses with the separate power wire. That is why those vehicle have different fuses for the tail lights and trailer lights.

expert reply by: John H