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The HVAC controls both the fan speed as well as the air temperature and air flow location. All models had manual A/C, although a digital dash option on some GTA's used an electronic servo to control temperature. On typical models, a switch controls the speed of the blower motor through a remote resistor. A mechanical cable would control the blend door, which balances air from the heater core and evaporator, to get the temperature you need. Full cold settings keep the door open only at the evaporator, full hot settings keep the door open at the heater. The airflow position is controlled by vacuum diaphragms throughout the venting system. Vacuum from the engine enters the HVAC controls, each position of the switch allows vacuum to flow (or not flow) to a vacuum diaphragm, which opens or closes a vent. If your system is stuck on defrost (the government mandated default position), it's possible you lack the vacuum to the control unit, with either a bad vacuum line or check valve at the vacuum inlet.