[Bruce Edwards]

The park is a function of an internal switch. It is closed when the wiper starts to move. When you shut off the wipers the switch stays closed until the motor comes around to the park side then the switch is opened. All of this is done inside the wiper motor. Sounds like the internal switch is sticking. You will have to take the motor apart to fix it. If you get lucky it may work it self out as you use it.