That yellow line is the vac supply line for the door lock system. Just to be sure that it is in the line, pull the door panel and find the solid yellow line again, and plug that end and use a MityVac to check to see if the line can hold vac.
In my car there is a vac check valve on that yellow line. I don't think you can get to it with out the dash out. I replaced mine when I had the dash out to replace the climate control accuators.

Check Valve - http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1L00T87R01VU0UGVXW&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1982&product=O4040-32232&application=000023991

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