Try replacing the temperature control switch for the rear control cluster. Hope that that's the fix. Less than $20.
if this system uses dex-cool flush the system
When you have the heater on, does it just blow any air at all? or no air?
The front expansion valve is clogged.
get a new heater switch
Defective blower motor Resistor Pack. Replace it.
Heater core may be defective
check heater fan to see if it is working.
Check your anti-freeze levels
Low coolant in the system. Defective heater control valve. Clogged heater core.
Likely heater core. I had same problem with 1996 S-10 with 4.3L.
the accuator, located in the rear passenger side is bad. the controls in front when turned go to an accuator in the rear there are two of them one for hot/cold and one for the vents.
Low on coolant. Bad thermostat. Plugged heater core. Temp blend door inop.