I just spent 2 hours looking for this piece.
I have the 3 fat MBZ manuals but they don't know where it is either. The only place it seems to be mentioned is in the wiring diagram and it's called: "Plug Connection. Tail lamp cable harness. "
I searched the trunk completely, took off the big plate behind the seats, pulled up the back seat, looked under the two front seats and in the engine compartment.
Wwwwwwwwwhhhhhhhheeeeeeeeeeerrrrrrreeeeee???????????????????????
TIA