sounds like you may have a short in the trailer hookup. If they work without the trailer and then blow the fuse when it is hooked up I would start checking either the connector or for a short in the wiring system. One time I put a 4 wire round recepticle in and when I used it the first time it blew the fused link on the battery. Turns out that there was a piece of one of the wires touching the recepticle. if yours is a flat 4 pin I would check all splices on the trailer and where ever you spliced into the truck. Just a thought.