Trailer lights don't work with headlights on

[pak]

I have a 2002 4runner, never had a trailer light problem before. I don't tow often, but just got a boat trailer and put new lights and wires on it. I am scratching my head as to why the trailer lights, all of them, work perfectly with the headlights off, but not at all with the headlights on.

I've seen some suggestion that the common (brown) wire might be shorted, but with new wires on the trailer, and rechecking the whole length, I can find no shorts.

Any ideas?

 

[Squeaks_&_Leaks]

I had a 3rd gen 4runner that had a similar issue. It took me awhile to diagnose.

There is a black (or possibly brown) single wire that is part of the trailer wire harness at the rear of the truck. Mine was supposed to be connected on the passenger side behind the plastic trim panel in the trunk but it was disconnected by the previous owner.

Use a multimeter. Connect one probe to the black wire at the truck's plug and connect the other probe to body ground. See if there's continuity. If no continuity, you may need to replace your ground wire or clean the ground point with a wire brush.