AC IN FRONT IS WORKING. BUT THE REAR AC/HEATER IS NOT. I TRY TURNING IT ON WITH THE FRONT AND BACK CONTROL AND NOTHING HAPPENS. WHEN I MOVE THE KNOBS TO NUMBER 3 I HEAR A CLICKING NOISE. ANY ADVICE WOULD BE APPRECIATED THANK YOU.:sunglasse

With or without sunroof?
Have you checked body PCM for codes?
What area is the clicking coming from?

I have the same problem, my 2001 tahoe (w/sunroof) rear a/c does not work. same clicking sound is coming from behind passanger back seat. An I have no idea what Body PCM codes are.