blows only thru defroster

Your flapper that directs the airflow is apparently not working.

i had that problem a few weeks ago after my motor caught fire. but i knew what it was and fixed it in about 10 min. there is a vacuum line that comes from the vacuum tree and into the car to the ac selector. the vacuum tree is right on the drivers side finder next to the brake master cylinder. you will see A/C on the black plastic tree next to the correct vacuum line. follow that line until you see a break. it may even be off. the line goes inside the car where the heater core lines go into the car.

i have that problem, but its only half the time, it will blow thru the defroster, other times thru the vent (only a handful of times a while ago)