Did you use Plats ??

Use these only
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU21T0H5B5L&year=1997&make=MB&model=C-280-001&category=F&part=Spark+Plug

..and change the coil to plug connectors [ the ones under the coils]