I got the NTG 4.5 audio 20 unit no (command) since it's the cheapest system for this car, but I had to read a lot about retrofit rear cameras for W212 models because mine didn't come with one. Then I found out that it's not even possible to retrofit the original camera system with MB parts.
After doing some digging, I came out an aftermarket module on AliExpress that ought to make it happen. However, I decided against that as there was little information available on the subject.
Retrofitting the original camera from MB is possible with a command system, but it's a bit of a hassle and can be expensive.
I came up with this system, basically.
command system for MB's initial retrofit - The China wired kit -External screen-equipped wired kit - GPS-enabled wireless display
I went with the wireless Garmin option that includes a GPS screen because the original GPS (Becker) is so useless and expensive to update (100€). To be honest, I could have saved money by not buying the GPS screen alone because I could have used a decent GPS.
I appreciate certain aspects of this. One device with both a GPS and a camera. Very simple installation; there aren't many wires to go inside the vehicle. From the exterior, it appears to be OEM, and the quality is great to the touch. It will save you a tonne of time and effort without requiring you to alter your current setup.
Reverse side of this remedy. After shifting into reverse, the screen takes around two seconds to load, and the image quality might be improved. Adding another screen may annoy certain people, but it doesn't disturb me in the least.
S$100,-Garmin GPS
Camera from Garmin 150€
Feel free to ask any questions you may have on this Garmin system or any other solution, and I will try my best to answer them.