If your defroster is wide open all the time, you need to check the vacuum
hose by the fusebox in the hood... look for yellow thingy with vacuum hoses
attached to it... one in and two out.

Pull out the side with one in... is there vacuum? If yes, then you most
likely need to change the defrost vent actuator in the glovebox.

If the defrost vent opens as you accelerate, then you need to change the
yellow checkvalve.