Could it be that the parking mechanism got out of sync? Sounds like mabye the park mechanism is broke.
The two-speed, permanent magnet, three brush electric windshield wiper motor has a brush rigging that permits selection of low or high speed. When the control selector is in LO position, the common brush and the blue/orange wire brush are used, and the motor operates at low speed. When the control selector is in HI position, the grounded brush and the white wire brush are used. Current bypasses a portion of the armature winding, causing the motor to run faster. When the control selector is moved to the OFF position, the motor will continue at low speed until the park switch contacts open, signaling the motor to PARK and activating the depressed PARK mechanism which is part of the motor output arm.