System Test

Check that all in-line fuses or circuit breakers are operational. Turn ignition and defogger control switch to "On" position. Rear window glass should feel warm to touch after three minutes of operation. If glass does not warm, use a test lamp or ohmmeter and check for 12 volts at feed wire to window grid at rear connector. The location of feed wire connector varies. It will be found either near the left or right upper corner of glass or in the rear compartment by the rear seat back, and will be either a Purple or Black wire depending on model. If voltage is correct, and window is not warm, check rear window grid wires as described in this story. If voltage is not correct, check wiring harness, control switch or timer/relay.
Test defogger control switch for shorts and replace switch if it is shorted. Some switches must be removed from panel to perform check. Check system for correct operation. If system is inoperative install a new relay/timer (locations vary) and check operation of system. If system does not operate, locate and repair short in the wiring circuit.