First off, do you have a wiring harness for a trailer? If so has somebody sliced into the factory harness or is it a plug in adapter for the trailer harness? If the factory harness has been spliced chances are it wasn't done properly and you have corrosion problems like mine had. 9 times out of 10 people who slice the wires just cover the connections with electrical tape. My suggestion is re splice it but this time instead of using butt connectors, use shrink tube (
Grote P/N 84-5006 dual wall is the best). Trust me, I build harnesses for logging trailers, so you need something that keeps out the moisture, and this stuff works awesome. I've never had a harness I built come back with problems, and I never had a problem with my truck harness since I redid the wiring at the back.
Good luck with that and I hope that this helps.