You might not need to go inside the radio, these also used a trunk mounted hard wired CD player made by Alpine ( part of Becker ) This CD player was also found in some Japanese cars and possibly some Fords. There were quite a few Alpine and Becker versions of 1492 type radio.

The later MB radios got fiber for communication so that would be more difficult to make your own.

Here is an off the shelf plug and play converter. ( $ 65 )

MBZ-AUX Aux Input Adapter for select 1994-98 Mercedes Benz :: Auxiliary Input Adapters

More radio info

Becker Model BE 1492 (1692 similar)

https://www.k6jrf.com/MB_Bekr.html

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