To: OneAmongOthers-It will have to be in one of the hoses that goes to the heater core. If it doesn't have one then it must be controlled a different way such as a door in the heat/ac assembly under the dash that directs the air to either go through or bypass the heater core. If the picture you have is of the car in question (looks to me like a 67), I am sure it has a
heater control valve. On my 67 it sits above and towards the back of the right (passenger side) valve cover. Again, all you have to do is look at the heater hoses and you will find it. I just looked at the
shop manual on CD that I have, and it shows that the
heater control valve is closed when it has vacuum applied. So you will need to hook it up to a vacuum source to keep it closed. Don't be surprised, though, when you stop the car and start up again (like if you stopped for lunch on a trip) it will put heat through the vents for a while. That is because there was no vacuum to keep the valve closed when you stopped, so when you start up again it does send some hot water through the heater core before the vacuum closes the valve. I hope this helps keep you cooler!