if you get down on the driver side in the floorboard and look up into that floor vent with a flashlight you should be able to see the vent door, look and see if the sponge type black material is still on the door and if it is going all the way down with the air on. couple things it could be, usually its the padding thats fallen apart over the years. or the door is getting hung up by something, or the door actuator has gotten weak and it will not hold it shut