I have just had a look at the spare heater unit I removed from a car I dismantled.
All the air into the unit comes through the rectangular hole on the back, about 8" X 4". Inside the unit the various controls direct it to the different locations. Fresh air by-passes the heating element inside and passes directly to the fresh air vents. Heated air can be switched to screen, footwell etc.
The grill on the bonnet scuttle draws air into a void between the bulkhead as viewed from inside the car and that visible from the engine bay. It extends the whole width of the car and runs down behind the door pillars, draining rainwater out through the slot at the front 6" or so of the sill.
Some but not all models have a closeable vent to this drain at the sides of the footwell.
The system is both good, in allowing a generous passageway for leaves etc to get flushed out along with rain, but is also a weakness, because
a) It rusts the back of the rear face of the pillar where the hinges attach, unseen until the pillar collapses
b) The gap inside is partly formed simply from oiled cardboard, which decays letting in water, which runs onto the floor and then under the carpet, causing rust and also corrodes the fuses, which are carefully placed directly in the way!
c) If debris builds up water is directed inside the sills, which causes rust
d) Water leakage into the void from the wiper spindle seals and also the choke cable on carburettor models.
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