Vacuum element for main air flap, location - GF83.10-P-2125-01A
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.
The vacuum element for the main air flap (44) is located on the automatic A/C housing and is accessible through the air collection housing at the right rear in the engine compartment.