The COMAND in 2003 E class has no support for reversing camera. If your COMAND has the green video input fakra on the end, you may be able to add an AV interface (that pretends to be TV tuner), which will allow you to display the picture from a camera, but it won't auto-switch
Alternative is to change to COMAND NTG2.5 (the 2009 model year version) which does support reverse cameras and add an aftermarket NTSC camera ..
