Where is the ac/heater water valve??

[starbuckk]

Probably a stupid question but where is the water cut off valve on a 1968 C20? I seem to have located everything BUT that. I got a replacement valve but for some silly reason I can't find where the thing is located! I may not even need to replace it, since it is vacuum actuated and I've replaced most of the vacuum lines.

But I don't have any unconnected vacuum lines so I can't trace to the darn thing. This indicates to me that it isn't hooked up at all, which is probably why I"m always getting heat out of the vents.

I've got both of the vent actuators hooked up and working properly. I just can't find the water valve.