I "manualized" an '80 300D CC system a while back.
I controlled the vacuum modules for the air ducts with the existing vacuum valves, switching them on or off by applying +12v with a $1.50 toggle sw. Same type toggle switch is used to turn ac compressor clutch on. Heater core water is also on a sw.
Simple and, I bet I saved 30lbs in servo, vacuum lines and wire.

I hoped to document the procees someday but, its pretty obvious when you dig into the dash and see how everything works.