Might be an alternator issue. Check out the tech article that I linked to below; hopefully it'll help you troubleshoot the issue. If you have any more questions, feel free to post a Comment just below the article and someone will get back to you with more detailed info...Best of luck!

Mercedes-Benz W203 Alternator Replacement - (2001-2007) C230, C280, C350, C240, C320 | Pelican Parts DIY Maintenance Article


-Dmitry