Chevrolet Repair: no heat control in 1996 blazer, coolant line, heat control

Question
My wife's blazer has poor heat. And poor ac for that matter.  I put a thermometer in the vent, and I can not get the temp to change with operating the heat control.  Is this electrical?  What can I do test it? Where is the valve that controls the flow to the heater core? Thanks...Tom

Answer
Hi Tom,
There is no valve in the coolant line. Hot coolant always flows through the heater core.
The temp is changed by directing air flow either through, or around the heater core.
All the air flows through the A/C coil.
The doors that control the air are controlled by  electric motors.

Van