Have you checked out the relay? It works the up/down by reversing polarity of the wires to the motor when it throws. Might be stuck inside and causing your issue. The area it's mounted in is known to get water from leaky windshields. At least mine did.
Or maybe one of the feed wires is shorted to ground? It wouldn't be a problem when the wire is negative, the motor would work fine, but when the polarity is reversed...
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