How do you access the interior air distribution vacuum control, is it through the center console, by removing the glove box or by removing the lower dash below the glove box.
and where does the vacuum line pass through the firewall. I need to check for a leak. Thanks

Since the W126 is in many ways comparable with the W107 SL, I would guess that the interior air distribution control is placed behind the glove box, above the lower dash panel. The glove box is easy to remove and checking for the control here would be the easiest option.
The vacuum line will pass through the firewall at the right side. This has to be visible underneath the bonnet.

Most "80-"90 mercedes models have the interior-air-intake at the right side (behind the glove box), as well as the penetrations for vacuum lines.

good luck

vacuum lines
 

I discovered today that the vacuum lines can be accessed by removing the carpeted part of the dash below the glove box, then taking one screw out of the carpet on the side of the transmission tunnel and pulling back the carpet.