For 1990-2009 cars

The rear window defogger is controlled by the rear defogger relay through a high side drive control circuit. Battery voltage is supplied to the switch side of the relay at all times and when the relay is energized battery voltage is supplied to the rear window defogger control circuit. The rear defogger relay is controlled by the HVAC control module through a low side drive control circuit. Ignition voltage is supplied to the relay coil and when the rear defogger switch is pressed the relay control circuit is grounded and the relay will be energized.