My understanding is that all ML were prewired. The cable may have fallen further down into the body panel and you may have to fish it out.
Rodney, the MCS was not FO until '01. Check Benza's pub for model numbers.rudeney - 2/10/2005 6:24 PM
I am fairly certain that all MCS units use the D2B fiber optic bus. RODNEY
Yes, it should be pre-wired. Stick to the model numbers in Benza's pub for the MCS. Click on MCS in my link for model number info, links and install pics.cavallino - 2/10/2005 8:48 AM
Greetings all, I'm new to the forum and have a couple of questions about my '98 ml320. The car currently has a standard am/fm cassette, but my plan is to retrofit an MCS with a DVD Nav. system, per Benza's guide, as well as a CD changer. I thought it would be prewired for the cd changer (so that dealers would have the option of adding a changer after taking delivery), but I see no cable in the jack compartment in the right wall of the cargo area. Did models sold without the changer lack the wiring, or am I missing something? If it is not prewired, how difficult is it to wire in a changer?
Also, if it turns out not to be prewired and a hassle to wire it, am I correct that I could use the the Nav. DVD drive in the dash to play music CD's, but that I would have to take out the Nav. disc everytime I wanted to do so? Any help would be appreciated.
Do the pre-2001 MCS units also have a fiber port?Wolfgang - 2/10/2005 8:05 PM
Ah, great info, UncAl and Rodney.  Here are some more files lying around on the server:
The OEM Mercedes CD-Changer in a 98/99 M-Class works fine with the MCS from a model year 2000 M-Class. It's just plug and play. Instead of the changer one could also use the fiber port on the MCS for an iPod, and use the changer commands to control it.. 
Yes. http://www.whnet.com/4x4/MY2000_MCS_pins.htmlcavallino - 2/10/2005 8:20 PM
Do the pre-2001 MCS units also have a fiber port?