I got this found this under 77 Comet @ AutoZone, but the vacuum port points towards the front of the car. The arrow on the side point to the water flow coming in at the top port. Is in correct? Thanks. http://www.autozone.com/autozone/parts/Everco-Heater-Control-Valve/1977-Mercury-Comet/_/N-inkjnZ91sh0?itemIdentifier=142760_0_0_8679&vehicleIdForService=214026207
If your car is a V-8, you can probably get something figured out with an aftermarket valve. The V-8 valve is different than a 6 cyl valve on a Maverick. You can see the V-8 type valve mounted to the cowl just behind the air cleaner. The V-8 valve can be hard to find and they bring pretty big bucks on ebay.
it is open when engine is off. once you start engine and vac come up, it will close. if it's the wrong one, it doesn't matter if it's installed correctly...it will be...open...at the wrong time...