There is a switch in the wiper motor that tells it when they wipers are in the "park" position. If it's bad, the wipers will stop in the wrong place. You can check it with an ohm-meter. A little 400-grit polishes up the contacts nicely.
As for the relay mod, it works GREAT on the windows - with no risk to burning out the motors. You are basically replicating brand new door switches and giving the motors the voltage they are supposed to have. I haven't tried a relay mod on the wipers, but it should be more complicated because of the delay/low/high.