have no wire diagrams but a few thoughts. From the cab back the wire needs should be as simple as a 7 wire trailer cord running back there that's ALL

How was the truck built ? If it was built as a tractor it will have trailer wire at back of cab plus wire harness for rear of truck, this is where problems can occur as people use unique methods to achieve their goal of working lights.

Start at rear of truck follow wires and see where they run, and the connections, I think you will find it