Thanks, I read through that thread, seems like my next step will be to pull the cover off where the sensor is located and check the wires with a volt meter. I don't know what voltage to look for, but I'll take measurements. I think soon I'm just going to take it to an indie shop and have them take a look. Maybe re-initializing the GM will work.

A slight update, I cleaned the windshield (both inside and out), then I washed the car and left the wipers on. With just a little bit of water on the windshield, the wipers did the same thing they do when it's not raining: swipe twice, every 5 seconds or so. When I sprayed the windshield with the power washer, they came on full blast. SO the sensor IS working, sort of. I have to wonder if something else---maybe whatever is causing the wipers to "park" in the wrong spot---is the source of all this.