First, I want to thank you for the answers to the question about the headlights on my 1993 Sable. Now I have another question....where is the swtich that controls the air conditioning/heater vents? Mine will come thru the floor (under-dash) vents and the defroster vents but not the center ones. We've looked underneath the dash...does the whole dash panel need to be removed or is there a vaccum hose that controls this?

 

I had this problem on my 90 chevy Lumina Minivan. There ended up being a vacuum line off from my carbuerator. Does your cruise control work, as on mine the vacuum line controlled both the air controls and the cruise control. I spent a long time trying to figure out the problem when the answer was right in front of me.

Hope this helps.

 

Yes, this is likely a vacuum issue. Get a good diagram of the vehicle's vacuum system from a shop manual and look at the lines closely. One will be missing/off or have a hole in it in all likelihood.