Determining Trailer Wiring Function Assignment for Installing Tail Lights

Question:

My right rear tail light I just bought has 1 brown and 1 green wire attached to it which makes sense. But the wiring on my trailer has 2 brown and 1 green. I’m not sure how to rig this up. I tried connecting both brown wires from the trailer to the 1 brown wire on my new tail light but that didn’t work because I’m still getting no light coming on when I turn on my runner lights or brake/turn signal. So I’m not sure what to do.

asked by: Jared G

Expert Reply:

Wire colors are not a completely reliable way to figure out the functions carried on them. As you found out, the usual color/function relationship of brown wire = running lights and green wire = right stop/turn lights may not always apply. If one of your brown wires has a tracer on it (a line on the insulation that is a different color) then this may be an indication that it is for something other than the running light circuit.

You referenced a circuit tester # PTW2993 and using this (or the similar tester # PTW2992) will let you figure out what each wire does. Use a piece of wire to connect the trailer plug's ground pin to the ground on a fully charged battery such as the one in your vehicle. Use another wire to connect the battery +12V terminal to each of the other pins on the trailer connector, one at a time while you apply the tester. This will tell you which wire at the back of the trailer is connected to each of the 4-pole circuits.

You may find it easier to just replace the trailer connector. We have a 35-foot 4-pole wishbone trailer connector # A35W42B that has dual brown running light wires so that you have one for each side of the trailer; both connect to the running light pin which is next to the ground pin/wire as shown in the linked photo. All you need to connect this to your lights is butt connectors like # DW05744-5.

expert reply by: Adam R