I'm having the same problem with a used 2004 ML500. No sound from the radio or CD player. I would welcome hearing a solution.

 

Had a similar problem. Don't even know how to explain "exactly" how I fixed it, but it's in the tailgate! You take off the interior liner so you can access the inner workings. There are a few "black boxes" mounted in the tailgate/hatch. You can see one on the passenger side of the hatch that has a RF cable and an electrical connector going to it. I unplugged the connector and the radio sound started working! plugged it back in, and the radio stopped again. It's probably an antenna signal booster or some FM filter or such, but mine was obviously bad. I left it unplugged, and the radio works fine.