You absolutely need a wire diagram. Someone told me that the factory manual is posted on Mercedes' website now, or something like that. I'm pretty sure you can download it for free - I bought mine for 50 bucks 5 years ago before it became available.

Just trace all the circuits, and double check where all of the physical locations are (the diagram will tell you), and start from there. It will be tedious, but methodical work and patience will yield great results. I'm not sure if you are an electrical whiz, but I most certainly am not; but with a diagram, it's totally doable.