How to Wire a Trailer Light with 1 Wire

Question:

have old trailer, flat 4 at vehicle, the existing lights on trailer has 1 wire coming out, how do I hook up to get tail at brake

asked by: Dan

Expert Reply:

It sounds like the light on your trailer is either missing a wire or it was only designated for one tail light function. If the light does not look old or damaged, I would guess that it is just a clearance light and should be connected to the running light circuit. If this is the case, it should not function as a brake light as well because it was not designed to do so. The brake lights and tail lights run on different circuits and they were not designed to connect to the same wire on a trailer light.

I have attached a wiring diagram that shows how a trailer should be wired with 4-flat trailer connector.

If the light was designed for running lights and brake lights, then it should use a dual filament bulb. Check the bulb in the tail light. If it is a single filament bulb, then it is most likely a clearance light. If it is a dual filament bulb, then it is likely missing the wire for one of the circuits. If this is the case, I would recommend replacing the tail light with a new tail light. If you can send me a picture, measurements or part number to the light, I would be happy to see if we have suitable replacements.

expert reply by: John H