There is typically a valve mounted to the side of the air box.  It has a plunger type actuator on it and the cable that controls the AC vs HEAT function hits that plunger and opens or closes the water valve in the engine compartment.
Other controls such as HEAT and DEFROST and AC have lines going to the various vacuum motors to open and close the appropriate doors for that function.
While this is fairly general information, with some logical thinking and some testing, you can figure out what needs to go where.
Or, just check out a shop manual for your particular car.  The Ford manuals have all the vacuum schematics.  This is one for a 70.  Yours may be similar.  I can email some better pics of this if you want.

 

Last edited by MustangSteve (4/18/2015 9:56 AM)

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ok, you can also get color coded vacuum diagrams from scott drake that are a virtual necessity.

my guess is that you will have some leaking vacuum pods also but i can tell you this . . there are only 2 vacuum lines that come out of the firewall for the ac . . one goes to the heater valve and the other goes to a vacuum can that is mounted i the right wjeel behind the wheel or behind the battery tray.

it is quite helpful to have a might vac pump when checking these systems.



 

Last edited by barnett468 (4/18/2015 4:15 PM)

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heres a diagram for the 67 - 68 from drake but the hoses aren't color coded . . i just gives you an idea of where the vacuum pods are located.

http://averagejoerestoration.com/resources/mustang-wiring-and-vacuum-diagrams/1967-mustang-wiring-and-vacuum-diagrams/67vacac1.jpg

Last edited by barnett468 (4/18/2015 8:48 PM)