Remove the rear wiper relay. The relay works just like a fuel pump relay or any other relay.
Here is example of your relay system and how to test it: Remove the relay and turn the ignition switch on and the rear wiper should not operate. If the motor works with the relay out then seek out a priest as the motor is haunted. To test the circuit do the following:
Bypass the relay with a piece of wire, (preferably fused). This means: Remove the relay and put the piece of wire in place of terminal 30 and 87 in the cavity where the relay lives. Even if you don't use a fuse all you are doing is giving the wiper motor the same voltage it would be getting if the relay was closed. All electrical diagnoses should come from the relay. It gives you the input and output for the accessory. The input is 12v hot on terminal 30. This usually this is constant hot wire from the main buss line on the fuse box. The 87 terminal goes to the fan itself so that the output of this terminal will be a grounded motor waiting for voltage. The 86 wire on this relay is normally fed through the fuse box via the appropriate fuse and gets voltage when the ignition is turned on. The 85 terminal on the relay is the field ground and energizes the relay. This wire comes from the switch. When the rear wiper switch is turned on it sends ground the relay terminal 85 which closes the points of the relay and allow power to travel from the 30 terminal to the 87 terminal. This gives power from the 30 terminal to the appliance at the other end of the 87 terminal. The grounded rear window motor receives this power, begins to spin and operate the wiper. By the way relays have a failure rate of about .015% so the problem probably isn't the relay itself.Joe