The ac is not blowing cold air.The Repair shop said it in the dash. on 1999 GMC Jimmy

The receiver/dryer is cold. Compressor working. But blowing hot air in vehicle. need to know how to determine why air not cold. Took off glove box and lower dash. All diaphrams that I can see are working, change air direction and open flap for recirulation. What changes from heat to cold. Tempature control, diaphram change airflow from heat to cool? Thanks

Asked by for the 1999 GMC Jimmy
The problem would be with the air mix door or air mix door actuator. The air mix door would break at the attachment points and get stuck.