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.
with a minor 600+k