Take off the black panel on the inside of the tailgate to reveal the motor and wiring.

If your anygood with a tester make sure your getting a voltage to the motor when the wiper switch is turned on.

If its live but the motor isnt working the motors dead.

I've got a spare if you need one!

If there is no voltage check fuses (probably better to do this first before anything else!) and also check for a break in the wiring where the wires go through the tailgate into the body of the car - these are notorious for breaking up over time from the boot being open and closed over the years.