I have acquired a 1985 500 SEC Cabriolet Wide Body.
the aftermarketstereo system was stolen from the car before I got it.
this Becker Black box is mounted on the transmission hump where a
airbag module would sit. There are disconnected wires coming from it. The antenna wire connects to a small metal box not connected to the Black box.
would really like to know what this box is.
I will update what stereo was ordered with the car, but I think it was a Becker Mexico Kurier.

EDIT: Becker Mexico stereo cassette came in the car from factory

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.