Start with flushing both heater cores. The rear one may be plugged with crud.
I have the same problem with my 93 Chevy suburban also, and I found that the rear has a separate heater core located behind the rear right wheel well. From what I understand you have to locate it, and test it, and then flush it if the hoses are clogged.
Heater core changed three years ago,I changed Thermostat Heater still blows cool air.
The heater core is plugged. Blow it out with a compressor. If that does not work replace the heater core.
May need to replace blower motor.
checked antifreeze level???
It has a bad "Blower Motor Resistor" The fan switch itself is fine.
can you please clarify what you mean, do you mean that the front heater blows cold but the rear blows hot or do you mean that when the heating is on cold it blows hot? many thanks paul
The heater core is stoped up / MAYBE. Are the engine is low on coolant, Check coolant level in radiator.
Most likely the rear expansion tube or valve is defective.
the heater blows warm air when fan is on low but blows cold air when fan is turn to higher setting
Get the air out of your heater core.
The heater fan ONLY blows the air, it does not make the temperature of that air. Your heater temperature controls how hot the air is. Adjust your heater temperature for hotter or cooler air.