Most wiper motors have a "parking" switch on the wiper motor. This switch senses when the wipers are in the home position. When you turn the wipers off, if you notice, they will normally continue to operate until they reach the home position and then stop. If they don't stop at the home position, it could be because the contacts are welded on the parking switch, or the cam or actuating arm are defective. The net effect is that the parking switch remains on and will not allow the wipers to stop at the home position.
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