73 Dodge Dart heat control

If you have that manual, the info you seek should be in there. If you do not have AC then there should be no factory valve at all in the engine bay. If you have AC, there should be a heater valve operated by vacuum, and controlled by the AC/ heater control on the dash

Chapter 24: Heaters and Air conditioning

The first few pages up through 24-6 show the simple routing of the heater hoses direct from the heater to the engine on heater - only cars

Page 24-44 shows the vacuum schematic of the AC cars and depicts the vacuum water valve

If what you have is different, either your car is from a foreign land or someone has rigged up something none factory

I didn't look, but there might be some additional illustrations in the parts manuals, and some of those can be downloaded from MyMopar.com Factory parts manuals