ATC is not your "run of the mill" HVAC system. A genuine GM shop manual for your vehicle might be necessary to diagnose it. Temp not changing could be a faulty sensor, (the system has 3 or 4 of them) could be a wiring problem, a problem with the mode switch or the blend actuator. If the air distribution is incorrect, (where the air flow comes out) it could be a vacuum problem or a vacuum actuator.
If it needs the blend door actuator, most auto parts stores will either have it in stock or can order it. The Dorman part number for the ATC system is: 604-106 and it is NOT interchangeable with the manual AC system.