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The rear vents are either operational or non-operational...depending on what options you have on your car. If your car has 'active' rear vents, they are activated by a vacuum servo at the demand of the heater control system.
The non air cars had functional vents on them. They were controlled by a vacuum pod mounted near the doors. The AC cars did not have functional doors.
If you have a cable in the shift console, it operates the vent door down by the kick panel of the car and not the vacuum operated doors.
If you look at your floor board next to the shifter console you will see a vacuum pipe. This pipe ran from the shift console all the back to the vacuum pod. (#2 in the picture below).
Yes I had the heater control apart a month ago.. it is a vacumm line from the heatrer control that takes engine vacumm and depending on the heater setting ( i think Floor? does anybody know?) it routes vacumm to the rear diaphram and open up the rear vent...
nevermind I found it in the chassis service manual.. anytime the temperature setting at the heater control is cold, then the rear vent is open... the rear valves should close whenver you turn the temperature up.....