The MCS does not have any AUX inputs.  All the audio devices (such as the
rear-mounted CD changer) are connected via the proprietary D2B fiber optic
interface.  There is a company selling a simple audio-only interface to D2B
that would replace the CD changer.  I don't have a link handy, but you might
want to join the forum at www.mercedeshop.com and search there.  Most ML
owners that want to integrate an auxiliary audio device have used either an
FM modulator or a cassette adapter.  I installed a rear-seat DVD player and
overhead screen using a cassette adapter.  I do not have the NAV or in-dash
CD player, so I removed the blank cover inside the MCS and routed the wire
through the MCS housing to make it a more neat installation.

Look here for details on a pretty neat iPod installation (I believe this guy
used an FM modulator, but also put in a switch to kill the OTA antenna which
helped with RF interference):

http://www.keysolutions.com/M-Class.nsf