That same thing happened to me.....I worked for three days on it. I had checked the fuses in the fuse panel inside the truck and all were good. Turned out there is another fuse panel under the hood...big black box that says main load center or something like that. Took top off of it and there were individual fuses for each light left and right includeing trailer lights.....I looked and found a bad fuse there for the side of the trailer that I had been working on. I replaced that fuse and all was good again.