Well let me ask the Barracuda, 67-69,owners in the know.What are the two vents on the lower rear of both doors for? And why did the other A-bodies not have these vents?
Those are for the exhausters. If you open the doors and see the rubber flaps on them, those are the exhausters. They let the excess air pressure out of the inside cabin so the blower motor doesn't get "backed-up" with pressure... If you keep pumping air in and not let any out, eventually it will get full... You have to keep letting air out so you can keep pumping it in with the blower...
There are exhausters on every car made, some are hidden better than others...