I scanned some Becker Operations Manuals I had available to me. (two were mine, one I borrowed).
1) Becker 374 - Used in the S-Class 1979-1981
2) Becker 868 - Used in the 560SEL, SEC and SL in the mid to late 80s.
3) Becker 1402 - Used in the W126 (but not the 560s) in 1988 on, and W124s of that era too.
1) Becker 374 - Used in the S-Class 1979-1981
Becker 374 Operation Instructions « Classic Jalopy
The Becker 374 Mexico Cassette Vollstereo Reverse was the standard radio in the 1979-1981 S-Class in Australia. These are the instructions.
www.classicjalopy.com
2) Becker 868 - Used in the 560SEL, SEC and SL in the mid to late 80s.
Becker 868 Operation Instructions « Classic Jalopy
Operation guide for the Becker 868 Mexico CD player. This was standard in Australian delivery 560SEL, SEC, SL models.
www.classicjalopy.com
3) Becker 1402 - Used in the W126 (but not the 560s) in 1988 on, and W124s of that era too.
Becker 1402 Operation Instructions « Classic Jalopy
Instructions for the Becker 1402 Grand Prix electronic. Includes radio functions, auto reverse cassette functions and wiring connections.
www.classicjalopy.com