can't stand the heat

I've got a 68 911L. I can't seem to turn off the heat!!! The boxes in the rear, after the heat exchangers open and close fine but they still allow a great deal of HOT air to blow by. I can close the interior floor vents but hot air still flows through the defrost vents in the dash. How can I stop the hot air flow into the passenger area?

I have had that problem in the past with my early cars. Each time it was the heater flaps in the rear of the car. I had a set that appeared to open and close just fine, as you have indicated, but they had holes in them that I could not see. Because of these holes, hot air was filtering up from the heat exchanges. That is pretty much the only way you are going to get heat into the car. The ventilation system on the dash just directs the air coming into the car. Check the heater boxes again, you might have missed something. Good luck with this problem, but I have had this situation on two different cars and the problem was the same each time. It was the heater boxes and/or the cable leading to them.