This 1996 Chevrolet S10 came in with the complaint that the heater/ air conditioner air only blows out of the defroster vents regardless of where it is commanded. It is actually a pretty simple problem and strangely enough it is not an electrical problem. The most common place for failure is in the under hood vacuum hoses that attach to the vacuum reservoir.
If you look closely at the picture below you should notice that one of the vacuum hoses is flat.
You can barely see it but the shorter hose has a split on the underside of it, at the curve.
The vacuum supply hose is attached to the engine (V6, VIN X) at the PCV housing. Removing the breather elbow at the throttle body makes it easier to gain access to it.
I am using a large flat blade screw driver to push the hose off of the nipple. A little bit at a time and switching sides. I did not want to break anything.
The vacuum supply hose removed from the engine. Small hose at the center of the picture.
There are two sizes of vacuum hoses used for this repair, 5/32 and 7/32.
All new hoses attached to their respective parts.
This repair applies to several different models and years. I think it was for year models 1994, 1995, 1996 & 1997. It applies to Chevrolet S10 pickups and Blazers. Along with GMC S15 pickups and Jimmy. Hopefully some one out there will help clarify this for us.