For 1990-2009 cars

The rear defogger system consists of a rear defog switch, part of the HVAC control head, a rear defog relay, located in the left instrument panel fuse block (LIPFB), body control module (BCM) and a rear defogger grid in the rear window. The coil side of the rear defog relay is fed battery voltage from 10A IGN 3 fuse in the LIPFB. Voltage flows through the coil, through circuit 193 to the BCM where it is switched to ground when the BCM receives the signal from the rear defog switch.

When the BCM grounds the coil side of the relay, the relay is energized, and current flows through the 10A DEFOG LED fuse in LIPFB, and through circuit 293 to the rear defogger grid. The current flow through the DEFOG LED fuse flows to the rear defog indicator in the HVAC control head through circuit 683. If equipped with power/heated mirrors, the current also flows from this fuse to the right instrument panel fuse block (RIPFB) through circuit 683, through the coil side of the heated mirror relay, to ground through circuit 450 -- right mirror or circuit 550 -- left mirror at the right body ground splice pack. Battery voltage is fed to the heated mirror relay. The current then flows through circuit 267 to the mirror heater elements.

The BCM will keep the rear defog relay energized for approximately 15 minutes the first time the switch is pressed, and for approximately 7 ½ minutes if the rear defog switch is pressed again.