I happen to have a ML500 here, with nav of course. The disc does say"2001 Alpine Electronics, Inc" on it, so I must be mistaken on the Siemens connection. I may be thinking of the 98-99 add-on nav system (pod type).
If you are fluent in German, you can go to the Navtech EU site and make an inquiry about the disc/discs you need. I saw the different discs available for Europe, and none seemed to mention Mercedes or Alpine..........
http://www.navtech.com/UserOrdersServlet?action=listNavigators&continentId=EU
I am not sure of the model number of the MCS unit.
I've tried a couple emails to Navtech for you, I'll report if I hear anything solid.
Gilly