Rear vent, function - GF83.10-P-2105-01A
WARNING: This page does not describe the selected car, but rather 42 other vehicles, including the 1999 Mercedes-Benz S600, 1999 Mercedes-Benz S500, 1999 Mercedes-Benz S420, 1999 Mercedes-Benz S320, and 1999 Mercedes-Benz CL600. However, it is still accessible from the selected car via links, so may be relevant.
Function
Louvers in the rear vent open as a result of the pressure from the air flowing through the vehicle allowing it to flow out.
If a vacuum is present in the vehicle due to a window or the sliding roof being open while driving, the louvers close automatically from the force of gravity. This prevents exhaust gases, dust and moisture from being sucked in.