I have a 1984 Z28, I'm wanting to hook up some air. My question is that, I can't get the vents to blow. It blows from dash & feet but not out the vents. Can anyone tell me where to start looking & what to look for? Any help appreciated!

check the vent doors in the passenger foot area. make sure they are in place properly

Also check behind your hvac controls, if you have a hissing sound coming from there, chances are you have a vacuum leak either at the **** or one of the lines themselves.