Where is the vacuum air control located?

there is an a/c control programmer behind the glove box on the dash. i got one from the junkyard and it is still doing the same thing. i have 2 96 lss's so i am going to change the controler on the dash and see if that fixes it from the other one and keep my fingers crossed if not i'll change the programmer from this other one too

A loss of vacuum or low vacuum could be the culprit. When all vacuum is lost the climate control defaults to defrost. Check your vacuum line for cracks/loose connections. The line for the climate control runs from the upper intake manifold (rear part) to a tank on the lower front fender and then to the firewall pass thru to the climate control module behind the glovebox.