Hopefully I'm not piling on, but...could it be that either the Climate Control Push Button Unit or the Temperature Regulator Relay that sits above (I believe) the glovebox are causing the issue? Reason I thought it could be either (in your case) is because of the age of your ride; likely neither of those have been replaced or refreshed electronically. If the electronics are sideways, you can press any of the Climate Control buttons all ya want, and function likely won't be "correct" for the button you press---aka outside air doors could be staying in an open position, etc.

Just a thought; perhaps others can chime in regarding.