car electrical systems are a real PITA. I had a Camaro one that had a problem with the tail lights. I don't remember the exact problem but I do remember after several days of studying a book on the electrical system and tracing wires and taking reading somehow I came to the conclusion that a factory wire needed to be cut. I cut it and it fixed the problem.
So all I can say is good luck
how good is your German? There are big forums for each and every Mercedes model ever built here. I own a R129 Convertible and the Forum I'm in has a huge amount of expertise in it. Same for other models and the W203 is a real mass model. This is only one example: http://www.mb-treff.de/forum/board.php?id=16
I stopped working on cars myself, decades ago. I got a new tool that does all the work for me. It's called a credit card. Hans & Franz at my local Benz stealership are happy to do all my work, for something like $3,000/hour. LOL
But, probably the best place on the interwebs to ask this kind of question is here: http://mbworld.org/forums/e-class-w212-109/ There is a guy, "konigstiger", who knows everything. He is/was a MB super-tech and is insanely helpful. He's helped me more times than I can remember.
I know of and have fixed a ground problem on the rear lights "junction panel".
There is one on each side if i remember correctly.
The ground wire "burns out" at the connector and causes problems.
It can be fixed by replacing the connector and stripping the ground wire anew
or grounding directly on the fender.
Unplugging the battery for about 20 minutes will trigger a "reset" of some systems
and that might solve some problems with "false" messages.