An item that I would check is one of the vacuum actuated servos. Once you remove the glovebox insert, you will see it on the left. With the engine running, see if you can see it moving right after changing from defrost to any other function ( besides "off"). You can even undo one or both of the vacuum lines, and see if you get any movement in either direction..( the little rod ought to move up or down). If you are really curious you and take it out ( turn unit counterclock wise about 45 degrees and pull it out) and then gently pry the sections apart ( you will see how to do it once you look at it) inside there should be two springs and some diaphragms...if either diaphragm is shredded, you know what to do... .... Order them from here, of course!
btw.. there are two other servos that work in tandem for the recirculating air flap.. if the are sluggish to move, or barely move, then one or both is busted also.
PS: no, you cant just buy the rubber diaphragm, you have to buy the whole kin-n-kaboodle
hope this helps...