There might also be a ground issue where the power "goes back" through the reverse lights b/c that is where it finds a suitable ground. All sorts of strange things can happen then.
The wiring / connectors could be corroded at either end causing a bad connection even though everything is hooked up correctly. Or you could have caught the plug in something and pulled one or more wires.
Has this very setup worked before?
If you have a multimeter and know how to use it, you could check the individual pins of the connector at the truck side for +12V/ ground and see whether they are hooked up as specified.
