I had trouble with mine a few months ago. The motor was actually good, but it was seizing up in the bushing that goes through the glass.
You have to remove the arm (you'll need a puller tool, a few dollars at Autozone) to pull it off. Then remove the motor assemble on the inside of the glass. I cleaned up the bushing with a small round wire brush, as well as cleaned the shaft. I put come anti-seize on the shaft and reassembled it. It has been working fine.