Hi, John:

I'll try out the test you suggested on the door vents,
tomorrow.

I just went out to my 91 GL and with a flashlight peered
into the rear window vents from outside the car. The top
half of this vent (which runs the full height of the window)
has a flexible rubber flap over a rectangular opening that
is open to the interior of the vehicle. This opening, as
has been stated, is filled with a porous "foam" (filter)
material.  This "vent" allows air pressure that builds up in
the vehicle interior (i.e., from slamming a front door, or
from the pressure increase from having air vents open at
60MPH) to escape.  I imagine they help eliminate pressure on
passenger eardrums as well.

As for the A/C issue:  all outside forced air vents should
be closed when operating A/C. So the pressure in the vehicle
would be fairly constant and little "cold air" would be lost
out these rear window vents.   But that's just a guess, at
this point.

In any case, these rear vents are way kool lookin' !

- - George Chapman, 91 GL
    Lake Almanor, CA