if the truck has the original wiring harness, there could be several, if not many places causing your problem.
check all the wires for brittleness, even if it means un-taping the harness to get to inspect the charging wires.
it would also be a good idea to pop the terminals from the plastic connectors to check the crimps and the wire condition at the crimps.
as has been already mentioned, grounds are very important to check, and in function of the circuit.
wiring problems can be a mystery at times, but mysteries can be solved by investigating everything carefully.