In my case (1995 S10) it was as simple as the temperature control potentiometer. The pot contacts were crushed, God knows what breaks and shorts happened there. Replaced the pot and all works like a charm. 
Check the heater control valve. It has heater hose running to it and is located in the engine bay. It will also have a single small vacuum line running to it. When vacuum is applied (usually by the AC system) the hot coolant going to the heater core is shut off and bypasses back to the radiator. If this valve is stuck or the vacuum line is broken you will get hot coolant to the heater core constantly
SOURCE: heater control stuck blowinf hot air out vents
did you cheked if there is enough refrigrant ( freon ) in your tubes by measuring the pressure . if there is you may need to check your condensor its may be clogged and not letting enough air comes through to cool the refrigran
SOURCE: chevy 1996 s10 pedal position code?
Another name for it is throttle position sensor. It is usually on the side of the throttle body, usually black in color, has a 2 wire connection, attached with 2 screws. Needs replacing.
SOURCE: 1998 Chevy S10 pickup air condition compres is on instead of heat
First be sure that you do not have the controls set to defrost the windows since on just about all vehicles nowadays with the controls set that way the air conditioner will be turned on...even if it is not so warm outside. So...be sure that you do not have any of the controls set to defrost and that the heater is turned all the way on.
It sounds like though that the control for the vents is not operating properly. This is a combination of mechanical movements of the vent flaps and small "air motors" that operate the flaps. These air units are controlled by air vacuum taken from the engine compartment through tiny hoses. If the hoses get cracks, holes or come loose from their fittings then your vents will not move correctly so it is a matter of looking and tracing each one of the hoses to make sure they are fastened correctly, have not come loose and are not cracked or leaking. The problem can even be in the engine compartment where the hose(s) connect to the engine vacuum system...so it will take lots of looking carefully with flashlights etc....good luck.
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