wiper arm doesnt move but i can hear the motor running on 2000 Ford Explorer

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whenever i turn on the rear wiper the arm doesnt move
(2) Answers
ziptie is right, it's most likely the retainer nut that holds the wiper ARM on. Just be sure the wiper motor AND arm are both in the parked position before tightening the nut, if not it might wipe the liftgate instead of the window, then it may be a stripped shaft!
There is a plastic cover over the nut you need to pop up for access to tighten it up.