The little red tab or "AT Cover pin" is a 70 cent part, I usually buy 2.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1QC0RFPB41QS119AOS&year=1997&make=MB&model=S-500-003&category=J&part=AT+Cover+Pin
Paul
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98 E300