Heat doesn't turn off!!!

I think this is somewhat common but I don't actually know the reason why. I never looked into it.

Usually a vacuum leak at the control unit and it has a check valve that is why it does not cause driving problems. The real problem is that Ford did not use a water control valve to shut water off to the heater core. You can add a water by-pass valve that uses vacuum. I would recommend the Dodge valve used in 84-87 Dodge 2.2(Turbo) Charger. Your problem is the door is not closing in the plenum at the heater core. You can see the rod that is attched to the flap door coming out of the case about the location of the passengers left foot.