Mark,
If the heater knob, bottom right with a sort of wave on it, is properly off (anti-clockwise) any air that arrives should be outside air temperature and not hot. Perhaps in heavy traffic you would get some hot air from the radiator and the motor itself but not from the heater core. If you are still getting hot air with the knob completely anti-clockwise there may be a problem with the connection. For me, looking at your photos, it seems off but it may be rotated half a turn. Try rotating it fully clockwise and fully anti-clockwise. Wirewheel confirmed there should be 5 clicks.

I'm not sure exactly where the interior ventilation draws its air from.

Peter

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