Heater Cable
The temperature control cable attaches to a lever in the hvac box. It is fairly common for the tip of this lever to break off and the cable no longer controls the door that this lever is attached to.
When this happens the air blows thru the wrong vents rather than thru the dash vents when the AC is on. The cure is to buy a new lever and install it in the hvac box and then reattach the cable to it. It's a little tedious, but, small hands, and good eyes make it a relatively simple fix. The lever simply drops into a hole in the top of the box and moving it back and forth while removing the old one/installing the new one, will line up the slots so it comes out/goes in. GM part number 3037629
To see this lever, open the glove box door, depress the retaining clip and let the door hang all the way open so you can see thru the opening to the hvac box....look slightly up thru the opening as the lever is at the top of the box.
