Volks:

The vents that are aft (rearward) of the rear side-windows
are, to my understanding, are not scoops to force air into
the vehicle. They instead, suck. Yeah, suck the stale air
out of the vehicle.  On my 91 GL they also allow me to close
the driver's door (when all the  other doors and windows are
closed) by allowing a slight release of pressure within the
vehicle when swinging the door closed.

I've been able to clean the foam filter material with blasts
of compressed air. Carefully, of course.  (Granted the car
hasn't seen alot of dirty air; it only has 50K pavement
miles on it.)

My two cents,

- - George Chapman, 91 GL
    Lake Almanor, CA