If you have cold air, and it only blows out the floor vents, then the "nipple" replacment fix most likely will cure your problems. If not, then the vacuum feed or resevoir tank could be the culprit, but it is probably a collapse of the nipple(*) in the connector attached to the programmer case.
Here is the nipple fix procedures I used:
The violet colored line shown in the pics below is the vacuum supply line for the HVAC system. It runs though your firewall, into a harness cover (with other wires) where it t'* off to a black vacuum line that connects to the vacuum storage tank, in front of your passenger side wheel, underneath the car. A leak in this line or storage tank (black plastic pill looking tank about 8 inches long) may cause the system to take a long time to store up enough vacuum to operate your system. You may need to check that tank and the vacuum connector for cracks/splits/problems.
On to the "nipple" fix. First remove the plastic panel that is way underneath the dash, on the passenger side. Move the passneger seat back as far as it can go. Lay on your back and look underneath the dash. you should see a bunch of multi colored vacuum tubes going into a clear connector that is part of a black plastic box (the box is your HVAC programmer).
There are two bolts that hold the plastic box together. One on the middle front of the box and one on the bottom. You may not see the middle one, but you can undo it by feel. Get a flashlight and look at the box to get an idea of where the bolts are and what they are like. Undo the bolts (you do not have to take them completely out) and use the flat blade of a screwdriver to pry the box open (like a clam shell). Keep undoing the bolts until you can remove/slide the clear (and black) piece that the multi-colored lines are going to out of the box. There are lines on the other side of the piece as well, but they are different colors. Note which color lines up with which other color (make a diagram if needed).
The lines lead to another clear connector insdie the box that looks like this, but you don't need to take the box apart and get to this piece for the fix. You only need to cut the lines at the connector itself.
Once the piece is dangling free from the box, you simply cut each vacuum line (do it one at a time or use your diagram of the colors) at the connector on both sides. Get some new vacuum tubing (I took a small piece from one of the lines I cut to NAPA, to insure I got tubing that was tight, but not too tight.) and rejoin/reconnect the lines, in correct order, matching the two different colors (from the diagram you made) using the new tubing. I used a little WD-40 to help me slide the vacuum lines into the tubing.
Re-route the connected tubes through the hole in the black box where the clear/black connector used to be, retighten the bolts on the black box, enjoy air from center vents.
The finished product would look similar to this photo. I only used about an inch of new tubing on mine instead of the several inches shown in this pic.
More pics showing the programmer and connector for further reference