Do you want connect an MP3 player on your stock radio? You don’t need to install an other expansive radio.

I have seen on the SLK forum in Germany a perfect tool that you connect with the stock head unit. On that tool you can connect your MP3 Player with the ear phone connection. The tool you connect on the stock car radio antenna. The MP3 Player gives out the music on the ear phone connection then it came into the tool and the tool gives it into your car antenna (like the XM commander). Now you search on the FM frequency band on your stock radio if you hear the MP3 then safe it on one of your radio buttons. That is all. You can connect all audio units that have an ear phone output.

Here is a link for the installation of that tool. The instruction is in German but the pictures say all.

http://www.mbslk.com/files/artikel/thbmp3.pdf