I'd think a servo under dash that moves doors inside A/C and heat, defrost if you're talking automobile.
its a vacuum canister with a diaphragm inside it, the vehicles vacuum pulls the diaphragm back. usually the diaphragm is connected to something that moves, like a door in the heater or ac system or even an arm on a heater control valve or on the cruse control system that pulls the throttle back and holds it there. the old corvettes has this type system to open and close the headlights. other vehicles had the same type lights, mercury and olds. this system may be called something else in different areas.
