First, clean the contacts between the truck and trailer. A pencil eraser works great.
Then, check the contacts inside each of the jacks (plugs). If that doesn't fix the issue, you have to follow each of the wires in the harness to locate the short.
I would find all the grommets where wires pass through and see if one is worn.
Essentially, you have a wire shorting somewhere.
When I have that issue, I check contacts, grounds, connections, and pass through points.
I RIDE, THEREFORE I AM
COWBOY UP OR GO CRY IN THE TRUCK