The DIN plug overrides the radio in the same way that a cassette player does. Also, yes, the radio will still work with the DIN plug adaptor in place.
Back in the early 70s, Becker made a fairly obscure glovebox tape player ("Olympia") with a DIN plug, so it's the same concept, only updated. Talk to Ed Ebel at Becker at 888-423-3537 X207 for more info.
You'll need one more piece -- a wire with a male ending on either end that bridges the Becker DIN plug adaptor and the MP3 player/Discman. You can get them at Radio Shack for a couple of dollars.
To see a photo of the adaptor, go to Rainier Koenigs site. He's a popular German radio dealer/restorer who stocks this piece as well.
Douglas Kim
New York, NY
#018260