For 1990-2009 cars
Rear Defog Relay Cmd.: The scan tool displays On/Off. The body control module (BCM) will indicate if it is commanding the rear defog relay On or Off.
Rear Defog Relay Fdbk.: The scan tool displays High/Low. This input to the BCM displays the state of the rear defog relay feedback circuit. Low is displayed when the BCM is providing a ground to the rear defog relay control circuit.
Rear Defog Switch: The scan tool displays On/Off. The BCM receives an input from the HVAC control head indicating On when the rear defog system is turned On.