I only post this in this sub-forum because of my 2 12"s. But there's a flap in the trunk that kinda just keeps moving when my subs hit.
http://img13.imageshack.us/img13/3773/0420120953.jpg (http://imageshack.us/photo/my-images/13/0420120953.jpg/)

Whatever ^ thing is.
Sorry if its a bit big and low quality. :P

It is just an air vent as found behind the bars in most cars. Older cars had the same vents but inside the rear door arch, on C-pillars etc etc. The flaps are to stop water.

its ment to vent air when your AC is on fresh, so the air comes in via near your wipers, then exits the car here. also when you close your doors, this reduces the pressure build up in the car. if you close the rear passenger side door, you can hear the flap moving