A/C cars have a slot in the cowl which holds the cowl fresh air vent door. There is also a small bracket on the back of the cowl / upper firewall area.
The cowl vent door assembly drops down into the slot and is held down with 5 screws and a healthy dose of seam sealer. There are strips of foam on the front and back edges of the vent door assembly which seal against the front of the cowl, and against the side of the bracket on the back of the cowl.
With no vacuum applied, the vent door is held in the horizontal or open position by a spring. When vacuum is applied to the actuator, the door is pulled closed and blocks the fresh air inlet to the blower.
