[davepl]

Yes, I am a master of this topic by necessity, ran into issues on my truck. See the other thread for routing.

Long story short, however, they went from a "vacuum switched on" to "vacuum switched off" heater control valve and so needed a third port on the switch that is backwards from the cowl vent. In later years they got both vacuum solenoid and heater control switch on the same circuit, operating in the same direction, so they only needed the one signal. So it becomes a basic vacuum switch in that case, one in and one out.