Quote:Flow through ventilation.
Air comes in through the front vents, through the heater, or by passes it, then through the cabin & out through the vents when the windows are closed, like in winter. If the vents are removed, & no other air outlet is provided, the cabin becomes slightly pressurised, then nothing. Sort of like blowing up a paper bag. No air movement means no circulation of heat & fresh air. [so don't fart.]
Coupes have then down on the body just behind the doors & sedans have them in the 'C' pillar & they serve their purpose well.

Blocking off the vents in front of the windscreen messes with the ventilation in the same basic way. No air movement, no comfort.