I have a 99' E320. The headlights on this particular vehicle are a whole unit, the smaller of the round lights (the parking light) or low beam is not functioning. I suspected the bulb but after further inspection I noticed the filament was intact, however the wiring insulation was frayed and peeling I guess because of the heat. With the same gauge wire and a little know how it seems like it would be an okay decent job.However does my issue sound more like a wiring issue or a light bulb issue. The insulation is down to the copper wire, and it constantly peeling back. I am afraid of a fire, if the wires decide to rub against another and a fuse does not blow.I suspected a fuse initially but do not know where to look and usually more than on thing goes out not just one part of the headlight assembly.
Somewhere there is a short to ground, which is over-amping the circuit to that lamp. MBs are positive-ground electricals and a nightmare to fix unless you are German and exceptionally handy. You may be able to just splice in a new wire and fix the problem yourself, try this first.
I would recommend calling a used parts dealer and finding a new wiring harness. If that wire is going bad the others most likely are too.