Thanks for your pointers!
It's from an "official" hole, the one the heater uses to get ... warm air
into the cabin, that is also covered by a plastic duct. My heater valve is
brand new. I'll recheck that it closes fully with the knob at the OFF
position. If I adjust it OK and it still does that then I may try plugging
that hole and have warm air enter the cabin on the passenger side only. This
will only bother the non-existing passenger!
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