I found an old thread on the multimedia section.
You should have a 1432 three piece radio.
Head unit and 2 controllers in the trunk, one is the tuner and the other is the amp.
Head unit is dumb and just has the cassette drive and controls the tuner unit via a bus cable.

Radio error code!!!

These can be upgraded to aux input by Becker.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech