I believe the park function is internal to the wiper motor, based on electrical contacts that rotate with the motor/arm. The motor has a constant 12v (probably switched on with ignition via a relay), as well as a second input to tell the wiper motor to make one revolution back to the park position via a brief voltage, or to keep spinning via a constant voltage. This second voltage could be intermittently applied via a timer for an intermittent wipe.
You could put a switch on the constant 12v, which is probably via a relay, flipping this off would stop the wiper immediately where it is sitting (we've all done this in our daily drivers when it's raining and we stop and park, turn off the vehicle - lo and behold the wiper stops right there on our windscreen. As soon as we turn the ignition back on, it rotates back to the park position via the constant (hot in run) 12v.
If you are having an issue of the wiper parking in the wrong position, simply remove the wiper arm mechanism from the motor splined output, turn the wiper motor off so it returns to its parked position, and reattach the wiper arm to the splines exactly where you want the wiper to sit every time it parks.