I always thought that the exits were on the front of the door because
it's a low-pressure zone.
This would be due to the design of the door/body, not because of the
front-most location.
My 4-door early Jetta's air exits the front edge of the back doors, My
Scirocco's air exits at the back edge of the doors, A3 chassis cars' air
exits through flaps in the body behind the portion of the rear bumper
that wraps around the body, 1971 and up Beetles' air exited through
those small, cresent-shaped vents at the very rear of the passenger
compartment (Flow-Thru Ventilation was a big selling point that year),
and earlier Beetles and Ghias had lousy ventilation unless you opened
some windows.
In the case of the Jetta (and all A1 VW's), not to mention our vans, the
air in question would:
1) Enter the ventilation system at the very front of the passenger cabin,
2) circulate about the cabin,
3) flow into an exit point; in the case of a Vanagon, at the REAR of the
front door,
4) flow FORWARD (!), and
5) exit the vehicle.
The air can flow forward inside the doors for two reasons- first, the
door space is a separate plenum, not subject to the air currents of the
general cabin, and second, the cabin is a high-pressure zone, thanks to
the front-mounted air intake, and the air exit vents wherever they may
be located, are in a low-pressure zone, thanks to some thoughtful folks
in Wolfsburg.

On Saturday, July 27, 2002, at 08:52  PM, simon glen wrote:
>   Similarly,
> for moving motor vehicles, air always flows from the back to the front
> and
> this is the basic principle upon which all vehicle through-flow systems
> work.

Yes, the aperture takes interior air and vents it to the outside- but
where does the interior air come from, besides leaks? From the
ventilation system at the front of the cab.

> Even the little slotted aperture at the base of the front doors on
> later T2 (bay-windows) and all T3 (Caravelles, Vanagons, etc) takes in
> air
> from the cab interior and sends it through the door to the front of the
> door
> where it is expelled through the slot between the door and the A-pillar.