If it is the same as my car ( 2007 model, but still a W 203 ), it is located under a black plastic cover by the windshield, right behind the battery.
Open the hood, look to the left rear corner.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1XC0ZKQX928614A3KX&year=2003&make=MB&model=C-230-004&category=R&part=ACC+Cabin+Filter

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2007 C 230 Sport.