For those considering abandoning the 'heidelberg' stock radio, upgrading to
an later vintage blaupunkt will allow the connection of one or two auxiliary
inputs.  The Blaupunkt Reno Casette deck, useful for books on tape, has an
aux input that can accept an 800mv nominal audio signal.  The Thummer remote
gives control on the steering wheel and adding an mpd01 microdrive player
will allow MP3 playback through the Reno/Aspen series.  CD players that have
the DIN plug configuration on the back can also be adapted to MPD01 or AUX
inputs.

MPD01s are discontinued now and can be had very inexpensively.

With a 2GB microdrive you have your favorite MP3 library in a package the
size of a paperback book.

The older reno/aspen style match the styling and ergonomics of the
minimalist Vanagon dash environment.

Often available on Ebay for ~30US.

I use a Blaupunkt America Sirius unit and feed it directly into a Los
Angeles MP72 head unit with an Orlando faceplate (nice muted grey color)

The Malibu/Orlando/Los Angeles faceplates will operate interchangeably on
Malibu/Orlando/Los Angeles head units.

In a recent 1800 mile wander through the desert flowers, it was nice to be
able to listen to Washington DC traffic reports, East Coast Weather, Really
Bad Hip Hop, Nasty Gangsta Rap and other fine offerings while searching for
the truth, defined as Blues for a sufficiently local definition of "truth".

Your truths may be different, depending on locale...