I'm installing 2 headrest and a rear view mirror monitor from SAVV to my ML with DVD player next month but I want to be able to utilize my Bose sound system for the sound effect. I know I can alway use the FM module but I would like to hook it up to my OEM MCS BOSE sound system for better quality sound effect. Is there a AUX input for 03 ML's MCS Nav head unit? or FM is the only choice I have?

Nah - the MCS has no aux in, unlike most of the COMAND systems in other MB's. So the FM modulator would probably be the easiest way, although there are people (usually electrical engineers) who have opened up the MCS and tapped some of the audio wiring for an aux input somehow.