Mercedes started publishing current flow diagrams in the early 70s. I presume somebody finally realized that the wiring was just too complicated for simple questions like this.
If I remember correctly, my 280sel 4.5, a lesser cousin of the W100 has the ground wire in that loom come out under the dash and is bolted to metal for the dashboard. The idea being to have the ground wire be as short as is practical. I would expect the 600 to be done about the same way.
-CTH