For 1990-2009 cars

### Rear Window Defogger System Components

The rear window defogger system consists of the following components:

• | Heating ventilation air conditioning control module (HVAC)

• | RR DEFOG relay

• | Rear window that has the defogger grid

### Rear Window Defogger Operation

When you depress the rear defogger switch, voltage is supplied to the RR DEFOG relay coil and the HVAC control module also illuminates the rear window defogger indicator. Battery positive voltage is supplied at all times to the RR DEFOG relay switched input and the RR DEFOG relay coil is always grounded. This allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger. The HVAC module also sends a message to the driver door module (DDM) and passenger door module (PDM) on the GMLAN serial communication circuit to activate the heated mirrors, if equipped

When you turn ON the ignition and press the rear window defogger switch for the first time, the defogger cycle lasts 10 minutes. Further operation results in 5-minute defogger cycles.