This has been tested on a 2010 E550 Coupe (C207). It should also work for W212 and current C class models
Three buttons are involved: the hang up call button...
The 1 and the # buttons.
Press and hold the hang up. 1 and # buttons together until you enter Engineering Mode. I can just about do this with one hand if the shifter is in N or D.
From Engineering Mode you can also enter Engineering Setting. Do this by pressing Hang up, 4 and # together.