My car has a similar problem, I believe. The previous owner's response was to take a glass stirrer and put it partially into the open door, to keep it open. This stirrer is currently partially sticking out of one of the vents, so it's unfortunately obvious that it was jury-rigged, and also, that vent can't be closed now. I think that this has compromised the performance of the defroster, as now air is coming out of the center two vents at all times (if I remember right, air's only supposed to be coming out of there with the CC switch in the center or the left-of-center position). I'd like to someday go in and fix this, but I'm a bit apprehensive since I haven't dealt much with vacuum systems.
OTOH, now's a perfect time to do it since my car's down for the count right now anyways...hmm...