Vintage Cars: 1989 560sl, brown wires, fuse box

Question
how to add additional cigarette lighter socket for auxiliary electronics?

Answer
the easiest way is to run the 12V from the power side of the fuse box, that way you can have it always on or only on with key. Just ground the other wire to a known ground - Brown wires are ground in MB's    You can also buy a multi port plug to go into your original