When the rear window defog switch is turned ON, the HVAC Control Assembly sends a rear defog on request to the BCM. The defog on request to the BCM is a 12 volt signal through circuit #681 from a Manual (C60) HVAC Control Assembly, and an ON command over the serial data circuit from an Automatic (CJ2) Control Assembly. The BCM energizes the relay coil by grounding the rear defogger relay control circuit #193. When the relay coil is energized battery positive voltage is supplied fron the RR DEFOG fuse #48 through the relay switch and rear defogger grid feed circuit #293 to the rear defogger grid.