Suggest you remove the cd player - anchor screws are located within the air vents above it (whole air vent block is easily removed) and behind the ash tray underneath the player (loosen but don't remove gear stick gaiter and slide back) to take ash tray out. The player is then held in by two screws. Mercedes will do this for you but claim it to be a 1.5 hour job and charge you for such - job takes ten minutes! Disconnect the player and remove the top of it and recover your disc. Sounds like the laser assembly is either very dirty or unserviceable. If it can't be cleaned, a new assembly fitted should be about £250-£300. If you try a scrapped one you will struggle to find a decent working one. Sorry I can't give you better (and cheaper) news.
Ken
June 2009