Although the W211 Comand HU and the Nav Processor are both DVD based, they are two different boxes. The Nav Processor is located in the trunk. They are wired via the MOST opto fiber to the AGW, also located in the trunk. For all the trouble and the cost, you will be disappointed with the functionality of the Comand/Nav in the W211.jakozlow said:
I have this setup in my '05 SLK and I use a portable Garmin Streetpilot 2620 when serious navigation is called for. MB USA sell a version of the Garmin as add on.
You can read some related discussion such as this one:
http://www.benzworld.org/forums/audio-telematics-forum/1314596-navigation-system.html