i think the little doors that allow the air to flow through the heater/air conditioner are controlled by vacuum diaphrams. So I would look for a leaking vacuum line serving the control unit, or between the control unit and the heater/a.c. This is of course a hard location to get to - under the dash, or at the back of the eng. compartment.
If there is no vacuum, the default is supposed to be that the defroster gets the air instead.