Its probably just the output shaft seized in its housing as this happened to mine. But make sure its not just a fuse.
To remove the rear wiper motor, remove the wiper arm (10 mm nut), remove the plastic trim from the tailgate (M5 allan screws and carefully yank it free of the hidden plastic clips, undo 3 screws and disconnect the electrical connector.
With the motor removed, remove the c-clip and washer from the wiper arm end of the shaft (they are small and probably hidden under all the grime) clean up the dirty/rusty end of the shaft where the wiper arm usually fits with a file or abrasive paper, as this will make it much easier to drive the shaft out later.
Undo the 3 or 4 screws holding the white plastic cover on the motor assembly. *** Make a note of how the simple gear mesh ***
Drive the shaft out using a soft face hammer, use lots of WD40 or similar, you may have to drive the shaft back and forth and twist it about to get it out, use a drift when it disappears into the housing.
Clean and polish the shaft back to bright metal, grease the shaft and reassemble.
30 mins to 1 hr
New motor approx £80 quid most places
Hope this helps