I had the same problem. Try doing the trouble shooting from TM 9-2320-289-34 pg 2-46 to pg 2-50. Also look at the wiring diagram on pg E-23. The TM is located within the resource tab.
I changed out my wiper motor but still had the same problem, would not go to the park position when switched off.
I beleive its inside the wiring connectors to the motor. I tested that out by cleaning the connectors, all 3 connectors and it worked for awhile before it will have the same problem.
The wires leading to the motor are too short coming off the engine side firewall so I would try cleaning the contacts and wiggle the wires till you can get some results.
If all else fails try getting a used one from the local salvage yard. Prices are to high trying to buy a new or rebuilt one because you will probably have to pay for a core charge.