Interesting. The green wire used as an aux ground hints at a USA fix. MB or another Euro manufacturer would probably stick with brown wire for ground.
The lamp faults relate to either poor grounding somewhere, or possibly that the lamp monitor circuit negatively affects the intended ground path. (Pun intended.)
That unit is a simple pass through connector.
Mercedes w164 Tail Lamp harness REPAIR Kit
View attachment 93539
Added:
Assuming that the lamp socket is ok.
A cheap and easy copy of that fancy in-line connector would be to tap the brown socket ground wire and add your own aux ground. Saves time and money.
A Scotchlok tap would probably be perfect!
View attachment 93538
(Some people hate those taps.)
A simple test procedure would be to add a Scotchlok tap wire and ground lug. If the added ground doesn't correct the problem(s) remove it from the frame and isolate/tape it off until next problem.
Adding an aux ground may defeat the lamp monitor circuit if that is a concern. It depends upon what fails. (Losing the lamp monitor wouldn't be a concern for me. See below.)
vic
Off topic.
We had a 1991 Toyota Celica that we loved. Mostly very reliable. The brake lamp circuit failed. I traced the problem to a undersized burned connection within a brake lamp monitor module. The module was expensive so not worth the price for the older Toyota. I bypassed the module. Problem solved. It seemed ironic that the safety lamp monitor caused my problem.
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