The rear window heating element not only contains a heating element to enable the defogging of the rear window, but also contains an AM antenna for the vehicle radio system. The 2 separate systems are integrated through the radio antenna and rear window defog module.
This rear window defogging system imposes a time limit on the heating of the rear window in order to avoid excess load on the vehicle battery.
The heating operation is controlled using the heated-rear-window switch located in the instrument fascia. This is a momentary-contact switch, which toggles power ON or OFF when it is pressed.
The heating is timed to continue for 15 minutes and each new activation of the heated-rear-window switch resets the delay timer.
Heating the rear window is only available when the ignition is ON, engine running. It defaults back to the OFF position when the ignition key is switched to OFF.
Circuit Description
Control of the heated-rear-window is through the heated-rear-window relay R3. This relay is located on the fuse panel in the engine compartment and is connected in parallel with the heated-rear-window indicator lamp (DEFOG) located in the heater and air conditioner controls.
Pressing the heated rear window defog switch closes the switch contacts at X2-1 and connects battery voltage through circuit 539 to the body control module (BCM) terminal X2-12 via circuit 292. This increase in voltage at BCM terminal X2-12 is seen by the BCM as the heated rear window switch input signal. Power is then applied to the coil of the Defog Relay R3 from the output of the ignition control relay R21, via fuse F13 and circuit 539. Once the internal BCM timer is initialized, it connects the Defog Relay R3 pull-in coil circuit to ground via BCM terminal X1-18, circuit 681. This causes the relay contacts to close, supplying battery voltage from fuse F22 through circuit 2540 and circuit 293 to the radio antenna/rear window defog module A93. The ground connection for the heated rear window is via circuit 1850.
Defogging of the rear window can only occur when the ignition is ON, engine running. This defaults to OFF when the ignition has previously been ON and is switched back to OFF and back to the ON position.
