Keep in mind the wiper motors work by switching the ground. I've found the rear motor to be quite reliable. I would first examine all the wiring in the tailgate. I've seen cables snagged on things, broken, shorted. Verify that the harness running through the right hatch hinge is intact. There are also ground wires riveted into the hinges that break loose at their ends, in the middle, etc. You may need to create a new ground for the hatch. I do this by adding a wire from the inside edge of the hatch opening on the car's structure and running it to the side edge of the hatch itself near the weatherstrip. Use a stranded piece of wire. If you can find some of the type of cable with very fine strands, it will last longer. Leave some slack so it can rotate as the hatch closes. In short, verify all the wiring before attacking the motor.