rear window wiper won't move back and forth

Most likely means the NUT holding the arm onto the motoe shaft is too loose or the shaft is stripped. Turn the Window Wiper off with the key ON to make sure it goes to PARK position. Then turn off the ignition switch and THEN put the rear wiper blade assy where it should be when the wiper is OFF. Tighten the nut and try it again. hope this helps