For 1990-2009 cars

Battery voltage is applied to the defogger relay, through the DEF fuse, when the ignition switch is in the ON position.

When the rear defogger switch is depressed, a ground signal is provided to the defogger relay through the switch to G200. This activates the timer circuit and relay, which provides battery voltage through the closed contacts of the relay to the indicator in the switch and to the rear defogger. The rear defogger is permanently grounded at G401. Because the rear defogger is a resistive element, the defogger begins to heat when voltage is applied. Since the indicator is ground to G200, the indicator lights when voltage is applied.

The defogger relay incorporates an integral timing circuit. The timer circuit will terminate the rear defogger operation after 15 minutes or when the ignition switch is moved out of the ON position.