at either side of the rear window you will have a connector, one goes to the chasis the other should be at 12 volts (13.2) with the engine on. this can be checked with a DVM or ise a spare side light to see of it lights it! if not you probably have a faulty relay or wire, if it does reseat the connectors BOTH sides of the window and see if this works then check on the other side of the windows as the chasis connector might not be earthing properly.