Heat is controlled by the "Temp Door Actuator". This actuator moves the door which controls how much air is sent to the heater core.
I had to remove my glove box and tape deck (middle of the dash) to replace mine when it went bad on my 90 GMC K1500. The actuator is available from ac delco (which means you do not have to get it from a dealer). Go to
www.acdelco.com for nearest parts store or buy it from
www.rockauto.com
//2000CAYukon