Like he said, it is a reversible ground system. Quickest way to check it, is get a jumper battery and 2 wires. Unhook the wires coming from the truck frame, at the 2 wire posts, on the rear end motor. Touch a hot and ground wire from the battery to the 2 posts, doesn't matter which is plus or negative. The motor should move depending on which way you touch the wires. If it doesn't, reverse the wires and try again. If it moves this time, reverse the wires and see if it moves the other way. Most of the time if it runs either way, but won't reverse, the problem is in the contact switch under the cover. Make sure the driveshaft is in neutral, to keep the shaft from being loaded, stopping the motor from switching.
