What Is The Purpose Of The 5 Flat Trailer Wiring Harness On A Vehicle?

Question:

My 03 Tacoma had an after market tow package installed. I’m not certain who the manufacture was, but it has a 5 pole harness spliced into the vehicle harness that was plugged into a 4 pole adapter. Which brings question #1 - Why would there be a 5 pole harness adapted to a 4-pole recepitcal for the trailer plug? The 5 pole harness is white / blue / yellow / red / green. Blue, yellow and red are spliced into there respective colors on the left turn signal harness. Green is spliced into the right harness. White is not spliced into anything. Question 2 - I’d imagine that is should it be? My lights do not work, and I’d imagine this is the issue because it provides the trailer circuit a ground through the vehicle frame. Correct? Thanks

asked by: Tony R

Expert Reply:

I will start off by saying that the white wire most definitely needs to be grounded. I always recommend doing this directly to the frame of your vehicle as it has the most metal. This is almost 100% the cause for your lights not currently working.

Typically the 5 Flat, like the Curt # C57187, is added to a 4 Flat when someone is pulling a boat trailer with a surge style brake actuator that has electric reverse lockout. The 5th port will be attached to reverse lights and will send power to the actuator and allow the trailer to reverse without the brake being activated. It is usually a lot easier to just plug one of these into an existing 4 Flat than cutting it off and then splicing the 5 Flat in, so this will be why it looks the way it does.

expert reply by: Lazer S