I just put new lights on a boat trailer. The left brake light works with the headlights off, but goes out with the headlights on. Any ideas? reverse polarity?

 

I would say a crossed wire is doing it.Retrace the wires and ground,check your plug for the right colors going in it.Is it a 4 or 7 plug you have on it?

 

Yeah check the ground wiring. If your using a 4 wire plug make sure the white wire is grounding to the trailer's frame and the ground wire from the lights is also making good contact with the frame.

 

Thanks for the replies. I have a 4 wire plug, I'll have to recheck the white wire ground on the trailer frame.

 

It'll be a ground issue at the offending taillight. With the parking lights off, the taillight can ground by back-feeding through the turn signal circuit, and the turn signal can ground via the parking light circuit. That's why the turn signal and parking light can't work at the same time. When both need to be on, there's no ground, so they both go out.

-Joe