The short is intermittent, meaning it comes and goes. It may be a bare wire that touches ground only occasionally as it shifts around. Try nudging wires gently, especially in the areas where you recently worked (e.g., under the dash and at the back of the truck, etc). It is possible that by moving the harness around during your recent inspections you temporarily "defused the bomb." lol
Don't forget to take a look at the wires attached to the trailer socket. Check inside the socket for mud and rust, too.
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