if im not mistaken the W210 cd changer goes on the left side of the trunk.. remove the left trunk panel and the fiber optic connection and power cable will be tucken back in there.
as long as your old cd changer in fiber optics it should plug in and work.. just a matter of getting a bracket. but you will need to have the D2B ring coded at the dealer for having a cd changer (although it will work right after you plug it in without programming it.)