It is a European brender up two horse trailer with inertia brakes for towing with cross over. So no powered brakes. Just 4 pin

Coming out of the brown pin, this trailer sends brown to the left side running and tail lights and black on the right side. But they are joined just prior to connecting to the 4 pin.

The wiring is clear but something is causing tail/running lights to blow a fuse

I’ll take a fresh look at the trailer from that perspective. Its a horse trailer so a lot of the wire is run in the walls of the trailer. But I really didn’t examine whether I could pull new wires through the space. If the are free and I can snake new wire it might be worth it. The trailer has two side markers on each side plus a top front and top rear running light in addition to two dome lights inside. So that aa lot of wiring and 6 lights to replace. But….as you said….I might spend as much time trying to fix an issue with no resolution!

Thanks for giving me the kick to think about replacing vs fixing. Probably better in the long run!

Thanks for your help.

That’s what’s confusing me. I did separate the left and right side and confirmed that no lights share these circuits. Yet each one can blow the fuse as well as when they are wired together. Since none of the lights share these wires, I’m a bit lost. I figured it was something related to a ground or a screw through the cable before the wires split into left and right but I can’t find anything. Having a bad LED or a reverse polarity LED or a LED + Incandescent COULD make sense if they installed a kit of lights the same way on both sides.

Keep in mind, if I apply the battery without a fuse everything works but the fuse and the diagnostics don’t like those two circuits.

I’m going to measure amp draw on those two vs the yellow and green circuits to see if there is anything different. The yellow, green, brown and black all use the LED tail/turn/brake lights so for now I’m assuming the issue is elsewhere.

I’ve already removed the little working dome light and the rear running light from the brown circuit but it didn’t fix it. I’m going to remove the side running lights next which means that circuit will only power the tail light. I’m hoping I can isolate the issue.