[84LX89GT]

Defrost is a default setting for the heater system for when there is a vacuum problem. Did you check for vacuum from the lines at the vacuum motors (the drum things). There should be a fresh air/recirculate door for when max A/C is off/on, a condensor/heater core door for heat selection, a defrost door and a vent/floor door. Because of the fact that you can't change Heat OR the location that sounds like to me you don't really have a vacuum distribution problem, but rather a supply problem. Check at the heater control unit to see if vacuum is getting to the control unit (the thing with knobs you turn), if not trace that to the vacuum tree on the firewall. One thing it also may be is that there is a vacuum reservoir for low vacuum conditions (such as wide open throttle, heavy throttle conditions). If you have a large leak for the reservoir vacuum circuit it may cancel vacuum to the door supply system.
First i'd just check and see if vacuum is actually getting to the control unit, if not then check at the tree where the line is, if so i'd trace along that vacuum line and see if there are any splices/alternate paths it may follow. I forget off the top of my head where the reservoir is spliced into, but if you need the info i can look it up.
Good luck, keep us posted.