Trouble Shooting
Turn ignition "ON", then turn window defogger switch "ON". Vehicle ammeter should indicate current draw. Voltmeter connected between right and left vertical grids should indicate 10-14 volts, and glass should be warm in 3 to 4 minutes.
If no current draw is indicated, check power feed and ground at rear window. Check fusible link and control circuit fuse. If system is still inoperative, problem is broken window grids or inoperative control switch/relay timer.