#1 suspect because it's the most common would be the linkages on the actuator for the vent positions. It's on the driver's end of the HVAC box, which is just above the driver side of the transmission hump. Looks like this:
You must be registered for see images attach
The linkages are a pair of spring-loaded metal plates that slide; they're lubed with a grease that gums up over time and keeps them from moving properly. You can clean it off gently and apply some fresh white lithium grease. Also, sometimes the linkages fall apart, or the push nut that holds them onto the end of the flap can fall off. Worst case scenario, the end of the flap breaks off (seems more common in 1995-up trucks for some reason) so the linkage has nothing to connect to...that sucks, because now you're pulling the dash for access to fix this.
If all that is in good shape then the next suspect are your A/C controls, because the 1995-up units are a lot more problem-prone than the earlier ones. Sometimes they get damaged when the radio is pulled because the antenna lead very often will fall down and short out the exposed circuit board of the 1995-up style controls (1988-1994 are completely encased in plastic.) I'm not saying run out and buy some new controls, because they're not exactly cheap - but also beware of any used 1995-up controls because very many of them are already bad.
Richard
