Eric, Wait until is dark and put a light on the drivers floor, a torch facing the pedals will do, or a normal stand lamp, open the bonnet and look for any light coming through the firewall, no light, no problems. Next check the Blower fan cover, the big black plastic lid like a lunchbox in the centre of the firewall, check the lid for cracks and the state of the seal, these lids sometimes crack and or the gasket goes brittle and leaks, allowing hot engine compartment air to get sucked in.

If all's well there, it would seem the air is coming from the footwell vent in the centre console, it requires removal of the dash panel below the steering colum, four screws from memory and careful with the courtesy light that lives in the panel. You can then check it's operation as there is a open close flap there. That flap is mechanically operated from memory, not vacume, maybe another member could clarify that point. But open or closed, it still has to have somewhere to get the warm air, passenger with a bowel problem?
Hope that helps!
280SL