UNIVERSAL MEDIA INTERFACE INSTALLED
The old can interface is being phased out, but the US still has to deal with this stuff in the US. All other markets have the newer interfaces.
The main harness for the Audio 20 is not that different from the Command, besides the monitor cable and the GPS antenna, not to mention the Audio 50, which uses the same screen as the Audio 20, and still they offer the 518 option for the Audio 50.
On top of that, the first Ipod interface offered (the one that only shows through the instrument cluster) was available for both the Audio 20 and the MM Comand as an added option, and it was the exact same part, which indicates that both units share similiar input systems.

The main harness for the Audio 20 is not that different from the Command, besides the monitor cable and the GPS antenna, not to mention the Audio 50, which uses the same screen as the Audio 20, and still they offer the 518 option for the Audio 50.
On top of that, the first Ipod interface offered (the one that only shows through the instrument cluster) was available for both the Audio 20 and the MM Comand as an added option, and it was the exact same part, which indicates that both units share similiar input systems.
AZ82.85-P-0004CW Retrofit Universal Media Interface. There are multiple procedures called out in this document to complete the task.
The main harness for the Audio 20 is not that different from the Command, besides the monitor cable and the GPS antenna, not to mention the Audio 50, which uses the same screen as the Audio 20, and still they offer the 518 option for the Audio 50.
On top of that, the first Ipod interface offered (the one that only shows through the instrument cluster) was available for both the Audio 20 and the MM Comand as an added option, and it was the exact same part, which indicates that both units share similiar input systems.
Another important point to consider is that both of MBs key competitors (BMW and Audi) offer at least USB (BMW) or USB/Ipod (Audi) factory options in combination without having to choose navi.
On the other hand, you do have the UMI, which offers something similar. But there's no telling how technologically close to one another the 518 and UMI are.

But if you mean "they could have built Audio 20 to support the 518 kit", I agree, it is probably still a marketing decision, wise or not.
UMI is a completely different option by complexity, after all it includes a full navi processor to start with. Having the necessary control for the 518 kit from such a device is a piece of cake.
For example, the backup camera is not offered on the w204 but it is "possible" to add it by going into the secret menu of the COMAND unit
The Best of Mercedes & AMG
Mercedes has yet to release an aftermarket version of the parts for this, but perhaps they will soon?
In the meantime, I understand that a retrofit kit may be ready from the guys at comandonline soon, with the costs obviously much lower than the $1600 for the full UMI kit. I guess we just need to patiently wait for all of this.....
For example, the backup camera is not offered on the w204 but it is "possible" to add it by going into the secret menu of the COMAND unit

Anyway, I'd be interested to see someone try it (you never know)...
I have not done it (already because the C-class is about to be built at the factory now) but a couple of forum members have done it on their own.
Parts costs vary depending where you get them, officially you would need the new head unit, screen, wiring harness, GPS antenna and the blower with air pipes.
http://www.youtube.com/watch?v=6Fmcf25CV0E
note the date on the video. Option 518 is the UCI kit which is the same as what comes in the new 2009 ML/GL/R/E/CLS/SL. The C can have this added provided:
1. it is a MM unit (non MM units only work with the more expensive UMI)
2. The command has the latest telematics update which provides UCI support
3. The command is SCN coded to have UCI turned on in the audio menu
4. You need the parts: The UCI module, fiber cable, power connector with 12v/gnd/wakeup, breakout cable (the cable that sits between the UCI box and the glovebox) and finally the ipod pigtail.
go on epc and look at the 2009 ML/GL/R for the part numbers.
If you guys are interested in a UCI plug-and-play kit let me know and Ill get you guys a group buy price...
http://www.youtube.com/watch?v=6Fmcf25CV0E
note the date on the video. Option 518 is the UCI kit which is the same as what comes in the new 2009 ML/GL/R/E/CLS/SL. The C can have this added provided:
1. it is a MM unit (non MM units only work with the more expensive UMI)
2. The command has the latest telematics update which provides UCI support
3. The command is SCN coded to have UCI turned on in the audio menu
4. You need the parts: The UCI module, fiber cable, power connector with 12v/gnd/wakeup, breakout cable (the cable that sits between the UCI box and the glovebox) and finally the ipod pigtail.
go on epc and look at the 2009 ML/GL/R for the part numbers.
If you guys are interested in a UCI plug-and-play kit let me know and Ill get you guys a group buy price...
thanks
It's my first time posting picture. I'm not sure how to do it, so I try with one picture first.





Location of the screws of the panel below the glove compartment. Picture taken from above. Notice the hook.

The location of the screws that hold the glove compartment.





