I just ran into this issue on an '81 911 SC (wipers wouldn't park & intermittent didn't work).
Cleaning the contacts didn't help. The solution for me was a similar fix to the common combo (turn signal / bright lights) switch problem.
Simply, the #2 & 3 metal blades weren't making contact when the wipers were in the Off position. I bent the #3 blade forward a bit with some small long needle nose pliers and that fixed it.
