Guess what: if you leave your Volvo in a moldy barn kind of place for a few weeks, it will come out with all kinds of odd electrical behavior. (Please don't do this to your car! I thought that the garage where I stored my beloved 1985 240 wagon was safe and sound...)

One thing that happened to mine was that the wipers would start and not stop. Odd, huh? I opened up the relay and watched it behave correctly, so the wiper motor clearly had power from two sources. Turns out it does: when triggered by relay, it continues to be powered by both relay and alternate circuit until relay deactivates AND "parking" switch inside the wiper motor is grounded, indicating that the wipers are at the bottom of the windshield.

OK, so I took the motor apart and sure enough the little copper tab that should touch ground when the motor is at a certain position was totally encrusted, as was the contact surface of the cover plate. Cleaning these did the trick nicely.

Note that you don't have to take the motor off the car to do this - just use a torx bit to open up the cover plate while the motor is still mounted.