Alternatively...
Hi,
I've just fitted a double-DIN Sony unit into my 2002 C203 and it was relatively easy to do.
Quick steps are as follows:
Remove the ashtray & unclip the lower part of the fascia
Remove the air-vents at the top of the fascia (gently lever to one side and pull out) and undo the two torx screws.
Move the fascia to one side and undo the two screws holding in the existing stereo and remove the MB stereo.
Screw in a fascia adapter plate and then fit in your choice of nice new shiny MP3-tastic stereo (you might need an aerial adapter - £5 from Halfords).
There are probably other stereo fitting guide in the forums (I'm new here

) so do a bit of research beforehand.
Most new stereos will plug straight into the existing wiring without any problems. I fitted a reversing camera at the same time (£30 from Maplins).
Here's a couple of links:
Sony Stereo with front and rear USBs:
Sony XAV-601BT Car CD DVD AV Stereo Bluetooth 6.1" LCD iPod iPhone (REFURBISHED) | eBay
Adapter plate:
Mercedes C Class 2000-2004 Fascia Panel Stereo Surround Adaptor Radio Double Din | eBay
When you sell your Merc you can remove your MP3 unit and replace the old MB unit.
I've found that this is the best way to get a decent stereo in any car. Once you've purchased a good unit you can move it from car to car as needed.
Hope this helps,
Dobbs.