I am very sure that it is a direct wire to the rear wiper motor from the switch on the dash. If you are sure you have power going to the switch (from a fuse) and after the switch is engaged on the opposite terminal you have power then I would look more into the harnesses or wiring possibly on the hatch wiring from the cab.

This should have it's own fuse sepporate from the regular wipers as they are two different circits (I would think by reading what schematics I have). Are you POSITIVE you have power to the switch in the dash and after the switch??????