[PeteVS]

How old is the battery? Batteries have been known to develop internal shorts. Does the battery go dead if it's disconnected? (You should have one of those green knob thingies on the car so you can easily disconnect the battery overnight.)

Just re-read your post. Scratch what I said above. Almost everything in the harness should be "hot" polarity. If you're getting a circuit from "hot" to ground, there's gotta be something ground touching one of the wires. Double re-check for a trim screw or something getting into the harness. Does the truck have a dome light? If so, I'd call that a possibility.