For 1990-2009 cars

The rear defogger system consists of a rear defog switch (part of the HVAC control head), the rear defog relay, located in the instrument panel fuse block (IPFB), the body control module (BCM) and the rear defogger grid in the rear window. When the ignition switch is in the RUN position, the coil side of the rear defog relay is fed battery voltage from the 10A IGN 3 fuse in the IPFB. Voltage flows through the coil, through CKT 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 30A REAR DEFOG fuse in the underhood fuse block (UHFB), through CKT 1240, through the closed contacts of the rear defog relay and through CKT 293 to the rear defogger grid. The current flows through the rear defogger grid, and goes to ground through CKT 550. The current flow through the grid causes it to heat up.

The DEFOG LED is fed battery voltage through the 15A HVAC fuse. Current flows through CKT 41, through the rear defog indicator in the HVAC control head, and is grounded through CKT 113 by the BCM. The BCM grounds CKT 113 when it receives an input from the rear defog switch. CKT 193 is also the ground side of the rear defog relay, as mentioned earlier.

The BCM will keep the rear defog relay energized for approximately 15 minutes the first time the switch is pressed, and for approximately 7 1/2 minutes if the rear defog switch is pressed again.