I have a Mercedes E350 W212 .My question is how can I access the back of my Comand system to connect a reversing camera

 

You didn't say how old your car is and whether your system will accommodate this. Software is a part of the issue.
What does your dealer say?

 

you need to remove the wood trim above the comand. After removing the trim, you will need to loosen 2 screws on each side of the comand, then you can pull it out. Do a search, there are many posts on how to remove the wood trim to gain access to the comand