Wiring Trailer Reverse Lights Using Spectro Trailer Wiring Junction Box # 38656

Question:

I just need a sanity check. I have backup lights on my trailer. I used a blue wire from the backup lights to the J-Box # 38656. I used blue wire because I had a bunch of it left over I will not be using the 7-pole black wire to spply Aux power to my trailer. My question is, Can I attach my trailer Blue wire AKA back up lights to the black terminal in the J-Box and use the Yellow 7-pole wire backup light to supply power to the backup lights?

asked by: Irv

Expert Reply:

Power for your trailer's backup lights needs to come from the tow vehicle's reverse light circuit. You will need to choose one of the available circuit posts on the Spectro Junction Box # 38656 as the one you'll use for the reverse light circuit. Usually this would be the Auxiliary post at the far end of the box opposite the ground. Please refer to the linked photo.

It does not matter what color wire you use to wire the trailer (although using the conventional colors does help keep things straight), or which junction box post you use, so long as they are the vehicle reverse light circuit wire and the wire on your trailer for its backup lights (your blue wire).

The yellow wire on a 7-way is usually designated for the Left Turn and Left Stoplight functions. This wire on your tow vehicle, if it is wired in the typical manner, would not activate the backup lights.

The best way to confirm which wire carries the reverse light signal is to have a helper carefully press the brake pedal while shifting the vehicle into reverse, so you can apply a tester like # PTW2992 to the vehicle 7-way to determine which wire carries the reverse signal for your trailer lights. Trace this contact back to the junction box and there join it to the backup light wire (your blue wire) on the trailer.

The linked article on trailer wiring may be helpful.

expert reply by: Adam R