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There is a Park Switch inside the Windshield Wiper Motor Assembly. More than likely it has malfunctioned. It is possible that the Windshield Wiper Switch it self is not opening the circuit for the Motor Assembly to go into the Park mode. If you are pretty mechanically inclined and have the proper tools you may be able to diagnose which one is malfunctioning, If not I would suggest a good, tried and true Independent Repair Shop to diagnose whether the Winshield Wiper Switch itself or the Park switch is malfunctioning.
This is about all I can tell you about your situation.
Hope this helps you.