Air conditioning

[89Suburban]

I am adding Air to my 73 GMC 3/4 ton, but, according to the only pictures I can find in this damn internet, I am supposed to have a Plenum Valve, that I can't find on any heater boxes in junk yards. The orange tracer line that goes to the valve in the right-side air flap has a T and a branch that is supposed to go to that Plenum valve. Where can I find one, and where does it go?

See if any of these pictures help.


https://www.gmsquarebody.com/threads/inner-cab-heater-ac-housing-reference-pics.4809/