Depends on what brand trailer it is but over the years we have seen that the running light wire goes up the drivers side front corner of the trailer, across the top, down the pass side, across the back and then back up the drivers side with each running light spliced in. Alot of times the spliced wire will have a little copper hanging out of the splice which will obviously cause a short. You either have to start tearing the trailer apart inside or try and isolate the short at each light. We see alot of screws pincing the wire as well. Call me on monday if you like for more advice.
Mike Gardner
703 675 4479
mikeg@hillwoodsales.com