Thanks for the tip. A couple days after the post it was raining one of those misty rains and I gave the wiper a shot, and it worked. Later that day, after the humidity level dropped, I noticed that the when I ran the wiper it ran really slowly. The next day we had real rain and the wiper moved as fast as it used to. My guess is that the humidity allows current to jump a gap that it can't otherwise. Because it worked I decided against removing the trim and pulling the motor (why do it unless absolutely necessary?) but I did notice a small gap in the sleeve that goes through the glass and sprayed some WD-40 in there. I haven't had a problem since, but I am quite sure that this will appear again.