On my 450SL it was behind the glove box liner. I could see it from underneath, but it was easier to see by removing the glove box liner. It was the one farthest to the right (passenger) side on my 450SL. On my 450SL there were 7 or 8 vacuum switches/controls behind the glovebox.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C