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I haven't had the problem, but something like it happened to someone else on the forum not too long ago. I think their problem was in the umbilical cable.
First, inspect the cable for cut/crushed spots. Also inspect the plug to see that it is intact and clean.
From there, look for the other end of the cable. It probably comes up through the floor under the couch/furniture in the front of your trailer, and goes to a plastic box. Inside the box, each of the wires goes to a terminal that connects to the trailer lights/brakes/whatever is on that line.
If you are lucky, the problem is in either the cable or the box at the end of the cable. The cable can be replaced without too much trouble, and the wiring in the box can be fixed. If the short is in the walls, that is tougher.