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Could be different problems. The blower uses a resistor to regulate fan speed, the resistor could be faulty. You would need to know if the radiator is full of coolant and if both heater hoses are getting hot. There may be a temp control valve in one heater hose that is not opening up to allow flow thru the heater core.
Your closing in. There is a motor that will open the heater core venting to allow it to blow into the car. Get a heater core install picture from your auto parts dealer or car dealer for specs of where and how it is driven.
Your AC compressor is activated via Climatronic control. The system regulates inside temperature activating the compressor or opening the flap from the heater core. This will be automatic when you set the temp you desired inside the vehicle. Unless you see the words ECON on the right side of the LCD display, your AC compressor will be engaged and the rad fan will operate to prevent the engine from over heating. Press ECON until you see it displayed and it will disengage the compressor and use only the interior blower fan to regulate the inside temp by opening the outside air flap and not from the AC.
you may have a failed blend air door motor/actuator. this part will regulate the amount of air from the heater core depending on the temp setting on your electronic climate control unit. if it has failed, it will be stuck at the temp that it was last set at when it quit working. it is also possible the heater core may be plugged up, but using the gm recommended dexcool coolant and flushing it out every 5 years, it should not be plugged up. you can pull the 2 heater hoses off the heater core on the passenger side of the engine compartment firewall and see if the heater core is the problem. caution-do not use a high pressure air hose to blow through the heater core as this could damage it and cause a leak!
Cheak to see that the coolant fan is operating with the A/C turned on. The fan has to work to help the engine stay cool and help the A/C. When you apply heat on the control it acts to reduce the coolant temp. You will have to get the coolant fan operation checked. This can be :fan motor, fan relay, temp sensor and computer. Good luck