Picked it up Saturday, was driving home yesterday in the rain admiring the rain-sensing wipers when we noticed the rear wiper motor was operating erratically. Did some research and discovered they have a current cut-off switch that senses when the motor is overloaded. The circuit will open upon an over-current condition, then close every few seconds in an attempt to clear the obstruction and at least store the blade, so it would move a little then stop... repeatedly.

Parts store wanted $150 for a rebuilt motor, stealership wanted $300 for a new one. Decided to tear into it and, after completely disassembling the brush motor and gear system I determined that the shaft connecting the wiper arm to the gear system was locked up. *Gently* removed the shaft with a punch and hammer and found the two bushings full of rust. There is an o-ring on the outer portion of the shaft which is supposed to prevent water from making its way into the shaft tube, but inevitably it gets in there. Cleaned up the bushings with some PBlaster and wire brush, cleaned up the shaft, applied anti-seize to the bushings and reassembled. Worked like a charm! We now have an operational rear wiper, and saved several hundred dollars. Figured this might be helpful for you Volvo owners out there, and it sounds like this is very common with their rear wiper motors.