I just fixed my 96 today with the same problem.....air blowing out defrost & floor only. There is a black box right above the gas pedal with several colored lines coming from it. This is called the vaccum programmer . It controls which doors open up from your control on the heater, ac control..
ok back to the black box...I found the easiest way to work on this without breaking you back is to remove the drivers seat and disconnect the electric on your seat if you have a power seat'''only 4 bolts holding it on.This gave plenty of room to lay down and look up.After removing the the seat, get a flashlite or droplite and lay on your back and look for the black box right up over the gas pedal Look for the green connecter which your colored vaccum lines are coming from.make sure the green thingy is tight against the box and all the vaccum line are intact, put a bunch of pressure on the connecter and sqeeze hard and at the same time have someone work the heater controls . Start it up and set on auto-60 degrees and see if air blows out the dash..Some times the problem is a loose connecter..they are real flimsy..
Unfortunetly on mine my programmer was shot and I had to replace it with a new one...PART # 16227376 FOR MY 96..It was expensive too....$1059.38..I got mine from a freind from a chevy dealership that happens to be parts manager..got it for $582.66 plus a $25.00 core charge. I think they are cheaper for 1995 and earlier models...
Before you start anything check to see if there is anmy codes that come up on your heater ac controls...To check for codes do the following..........
1/........push the up and down fan button at the time and wait till the ttemp goes to 00.and then release and hold the fan button in between the arrows and see if any codes come up.....If codes come up then you will have to find out what it is....maybe on the web or dealer....or parts store.......
I hope this will help you out.............john............MY 96