There is no adapter.

You can purchase one of the cassette adapters (which work very well unless you still want to play cassettes), but that will still require removing the HU and routing the wire out so you can close the door (if you have the newer-style Becker).

Or you can remove the HU and ship it to Becker and for $100 they will install an adapter that will allow you to plug in any standard 3.5mm audio input. (There are instructions on how to do this at Benzworld, but if the info here is over your head, soldering on the PCB in the HU probably isn't right in your wheelhouse.)

Good luck.