There are vacuum pods and lines under the dash on a 126 car. You may get lucky and see part or all of the problem by removing the glove box and looking inward from there.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/part.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&cookieid=1KU172UOY1M10SEPCZ&baseurl=http://catalog.peachparts.com/&year=1987&make=MB&model=420-SEL-001&category=O&showChildren=false

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"