Same here, solution.
My wipers do the same thing, and have done since I got my '94 D1.
I spent hours trying to solve this problem. The only solution is to replace the wiper motor (so I've been told)as the park switch is a integal part of the of the motor. you can see the switch it attached to the side of the motor. Unfortunatly you can't just buy a new motor. The motor and the mechanics are one piece, costing around $200. Money I can put to better use else where on the truck. I 've also gone down the road of finding used units and both sent to me where dud too. so I've given up. (lesson when asking if a part works. Ask a questions that have a more detailed answer)
If your are lucky enough to find a repair, please post it I've see a couple of threads here and else where from people with the same problem. Your not the only one.