The last time I had a problem like that, I f***** up my turn signal switch on my truck trying to correct it. That was a $500 mistake.
I would suggest tracing all the wiring on the trailer and making sure there are no shorts and if nothing is found, get a new trailer harness and replace the whole thing. Mine ended up being two wires that were spliced incorrectly in a part ot the trailer I could not see. Worked fine on another truck, but on my new one not so much.