A 2004 W211 does not support a reverse camera. If you had TV option you could connect one to of the TV Aux inputs and select that manually when you wanted to see what was behind.
If you changed the audio system to a 2009 Comand NTG2.5 then it would support a NTSC camera (and select it on reverse automatically) and you could use an aftermarket camera (there is no factory one or cut-out for one on a W211)
