1998 corvette, A/C blows out the defroster but not

I think you have a vacum problem. Check the vacum hose that, I think starts at the rear of the intake manafold
that hose will pass through the fire wall. Check to be sure
that the hose is not split or BENT stopping the vacumn
from getting to the A/C & Heater system.
Part of the job of that hose is to open & close the vents
air direction.