This document describes the various reversing camera solutions used by Mercedes Benz in their vehicles. This does not apply to Japanese spec vehicles.

History
Mercedes first introduced reversing cameras in the W221 S Class (and W216 CLS). This was followed by the W164 ML/GL and W251 R class vehicles with Comand NTG2 (i.e COMAND pre Model Year 2009).
NTG2.5 COMAND was then introduced and that supported reverse cameras properly, but the only vehicles that came with NTG2.5 COMAND and a factory reversing camera were W164 ML/GL, W251 R class and the Viano / Vito with the single panel rear door.
With the introduction of NTG4 COMAND, reversing cameras became available for the W204 C Class, W212 E, W218 CLS and R197 SLS vehicles.
NTG4.5 COMAND also supports reversing cameras, and it seems that all new models (such as the new W176 A, W246 B, etc) can have reversing cameras.
With the W176 A, W246 B and the other "small cars", there is a version of the Audio-20 that supports reversing cameras too, but it is different to the standard equipment Audio-20 which does not have the necessary internal hardware, and unless the vehicles have certain options the required CAN (car network) connection is not present which makes retrofitting reverse cameras to the small cars slightly more complex.

Cameras with Lines or Not?
The guidance lines function that exists on some cameras has been implemented in lots of ways.
On factory fitted W221 S and W216 CL vehicles these are supplied by a separate control unit. There used to be retrofit solutions available with these control units but the only cameras available now are basic and have no lines. These cars have active lines - i.e they move with the steering wheel to guide you into place.
The W166 ML/GL camera has active lines too, but these are generated by the camera itself.
The W164 ML/GL and W251 R class have static lines which are superimposed by the NTG2.5 COMAND unit when enabled.
Before about MY2014 on E/CLS vehicles, the cameras have no guidance lines, however MY2014 onward used the W166 ML camera and have guidance lines.
The W176 A, W246 B and other current small cars, along with the R231 SL also have the active guidance lines as they use the same camera as the W166 ML.
In the case of the Viano/Vito the guidance lines are static and generated by the camera.

Camera covers
As of late 2013 Mercedes introduced a camera cover that opened when reverse is selected on the W166 ML/GL. This was also added to the W117 CLA, and I expect it will appear on other vehicles too. This stops the camera getting so dirty.

Guidance Lines alignment
Special tools are needed to align the cameras, along with Star Diagnosis. In the case of the W221 S Class you can make this from cardboard. In the case of the newer cars it is a large T bar metal piece to set the distance from the wheels, a special pattern and a laser device to align and it costs about £1000.

Non Mercedes cameras?
There are lots of non-mercedes cameras available, often very cheaply - but the cheap ones are very inferior when compared with the exceptionally clear (and good an night) Mercedes genuine cameras.
You do need to purchase a NTSC camera rather than a PAL one if buying any-old aftermarket camera to fit.

Option Codes and compatibility
The Mercedes option code for reverse camera on the passenger car series is 218, and on the Vito/Viano is FR8. The code for the camera cover flap is XXX.
If it was possible for your vehicle to have had reversing camera the correct way to configure the COMAND (or Audio 20 in the case of the newest cars) to enable the reversing camera functions is to have the option code added to the vehicle's datacard and then SCN code the COMAND. Your MB dealer will have to do that though, or a good independent with a supported Star Diagnosis system.
On NTG4 / NTG4.5 it is possible to turn on the function using Engineering Mode, that is the subject of another article. With NTG4 if you enable reverse camera you can not use the Media Interface video input as it is the same input.
With COMAND NTG2 only the W164 and W251 COMAND units can support reverse camera in their software, and not all have the required green single fakra for the video feed on the rear. The other NTG2 units can not be made to switch to the camera video input when you select reverse.