The park switch is part of the motor itself. Or it's on the pulse board. Lemme check the EVTM for a 1991 ...
OK - it's not on the pulse board (which the 1991 EVTM doesn't show one!), for the Gen1's it's part of the wiper motor itself. Its function is to let the motors run until they're in park, so when it shorts, the unit will keep wiping.
The OTHER choice may be that the wiring is shorted around it.
There's a chance it's the interval governor, the module under the dash that powers the unit (if it's shorted, it won't see the switch open, so ... )
The signal is on the black/pink wire from the wiper motor.
Before you swap too many parts, I'd see if that line is grounded all the time or if it pops open when the unit is in the park area.
And if it pops open, but closes to ground when the blades go up very far, then it's not the motor. If it's grounded ALL the time, then it's the switch on the motor, and replacing with a later model (sealed) wiper motor may not be a bad idea.
RwP