In Alfred's picture above,The fresh air vent on the right has a metal tube that goes directly to the firewall. If the seal at the end of this tube (seal is foam) is deteriorated, missing, or the tube is out of alignment, It will leak air to the rear of the dash regardless of the lever position. You would need to remove your under dash panel and glovebox to inspect it. There is also a rubber seal on the intake scoop flap by the wipers that could be missing. This flap is controlled by the top left lever with the blue arrow in Alfred's picture above. It lets outside air into the vehicle. This is usually set to closed when running the heater in very cold climates. Think of it as a (recirc) button we have on modern cars. Same would hold true if you have Air Conditioning. Closed if very hot outside.
Tyler
