1999 GMC Suburban problem

I would suspect blend door actuator

Temperature door is controlled by an electric actuator, based on temperature knob position. When temperature knob is in cold (Blue) position, air delivered by system is unheated. When temperature knob is in hot (Red) position, all air passing through heater module is heated before it is discharged. Intermediate position of temperature knob results in a mixture of heated and unheated air to provide moderate air temperatures.
Disconnect negative battery cable. Actuator is located on left side of A/C-heater assembly. See Fig below. Disconnect actuator electrical connectors. Remove adjuster links from control lever. Remove screws and remove actuator. To install, reverse removal procedure.

good luck