Vent Stuck open.

no its not that simple as turning a knob...lol i wish. I was in the same place u were...i couldnt figure it out...i think i either posted or searched but i found the answer from a guy who had the same problem....

It turns out its a vacuum leak under the hood. Look for a black vacuum line that is the size of a pencil in diameter that runs behind the intake. It starts at a "tree" of vacuum lines on the drivers side fender and runs aross the top of the firewall. If u look right behind the intake, u will probably find a broken line. Go to murrays or your equal and buy some connectors to hook it back up....

The guy who told me had his break in the same place that mine broke in...which is right behind the intake. Im not sure what causes it but i'd bet this is your problem.

Edit:
***oh yea, the reason its stuck on defroster is because that is the default setting for the vent. Once u lose that vacuum, it stays on default and cannot change.***

GOOD LUCK

what he said