There is a switch on the rear hatch that will stop the wiper from working. It is supposed to prevent you from turning it on if the hatch is open. If you have the split hatch (tailgate goes down, glass goes up) there are two switches. You need to pull the trim panel on the tailgate to get to both of them.
If yours has the single unit there is only the one for the glass. You still have to pull the trim panel unless yours has the old style switch that sticks in the hole next to the latch.