sounds like your truck came with a factory trailer package, it should plug in at the back of the truck ,the main harness runs from the front , witch plugs into the tail light harness, the trailer harness is plugged in between the two harness's, might have to pull the tail light out to find it,the trailer harness witch they put in is probably faulty , just pick up another trailer harness that plugs in , or you can find where the connection is bad and spice in a new wire for that brake light