I've seen plenty of wire harnesses that have extra wires.
if your old harness was neatly tied off at that wire, I'd tie that wire off.
however, you could also go to a mercedes dealer and see if the wire is in the EPC...
in your pic, I see a wire directly below the one you are holding that looks like it would match, no?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!