From the 1967 Pontiac Service Manual: "With OFF sector exposed in panel window, no air flow or motor output will result".
Later cars did keep the fan on all the time - but not 67's. OFF was truly off. The little hose you are referring to connects the back of the motor to the air inlet duct to keep the fan motor cooled with relatively clean air, and has nothing to do with the heater core, and does nothing when the fan motor is not running.
If Pontiac meant to leave the heater door open a bit, then they should have saved the money they wasted on the foam seal around the door edge. And they also should not have shown the diagram with the doors going all the way shut in my service manual.
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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 Catalina Safari Wagon.
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