A number of years ago, I had a wiper problem. I don't recall if it operated only at one speed or if it was totally dead. In any case, the wipers worked fine during dry weather. But, wet weather (when you really need the wipers), they would freeze up. A tap on the motor with a Mag Light got them going again.
The fix was a new motor. I went to the Ford dealer and they indicated that a newer style motor would have to be retrofitted in. There is a technical bulliten on it. The harness needs to be spliced with a new connector as well. No problems since.