Ford Repair: Rear Wiper Motor, ford explorer sport, original equipment manufacturer

Question
I've replaced the rear wiper motor twice on my daughter's 2001 Ford Explorer Sport. The symptom has been the same on all three motors. The OEM motor sounded like it was running but the shaft was not turning. Thinking gear problem, replaced the motor with supposedly a factory refurbished one. Mounted motor and used switch to turned on, worked fine, turned off with switch, then mounted wiper arm. Tried again and all seemed ok. Few days later tried it out and the thing was doing the same thing as the OEM, sounded and felt like motor was running but shaft wasn't. Took motor back to AutoZone for a replacement. Replaced and the motor worked, wiper was wiping, everything ok, great! Few days later the exact same problem, motor sounded like it was running, felt like it was running but the wiper shaft was not turning! Called AutoZone and was referred to the Tech man, was told to replace motor and strap a ground wire to the motor case. Haven't done this yet but was wondering your thoughts.

Answer
Hello and thanks for the question...I'll try to help.
 OEM is Original Equipment Manufacturer...this is not indicitive of AutoZone parts. I would take a look at the motor and be sure that it is getting a good ground and also a good power supply. Then check the arm to make sure it isn't stripped out.
Erik