You have a computer controlled HVAC system ! It has the capability of setting DTC'S - diagnostic trouble codes !
The HVAC control module is a non-GMLAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive and ignition 3 voltage circuits provide power to the control module. The HVAC control module during defrost mode will change the A/C compressor LED status from on to off. The control head displays recirc, outside air, A/C and rear defrost feedback status through LED indicators. The control head has a rear defrost request switch please refer to Rear Window Defogger Description and Operation in Stationary Windows for operation. The HVAC control head will display NO COMMUNICATION when checking DTCs or doing a vehicle diagnostic system check refer to Diagnostic Trouble Code (DTC) List - Vehicle and Diagnostic System Check - Vehicle in Vehicle DTC Information.
The rear window defogger system consists of the following components:
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Body control module (BCM)
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Heating, ventilation, and air conditioning (HVAC) control module
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Rear Defog relay
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Rear window defogger grid
Your probably better off taking this to a ASE certified repair facility ! You probably have bad Heater / A/C control panel .
HVAC Control Module Malfunction
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