Tom Jennings, creator of Fidonet and owner of one of the first ISPs — Little Garden, is building a wonderfully-functional MP3 player for his 1970 AMC Hornet, using its standard-issue, 36-year-old AM radio as the starting point. His riff on a "proper interface for an automobile" should be required reading for all car stereo designers.
This is the American Motors Corp. factory AM radio in its natural habitat. Nice looking radio and all that. Not only is it a "shaft-type" radio, non-DIN, but it's very shallow; even cutting the center dash piece all to hell won't fit a modern stereo.
Look carefully at this thing, interface-wise. There's not much to it, but it gets the job done quite nicely.
Knobs on shafts that rotate. Simple! Fits the human hand and wrist just great. Very intuitive — infants figure them out (though infants usually pull the knobs off and put them in their mouth).
That knob on the left — closest to the driver — makes loud or quiet; that's all that it does. (The one function overload is volume-lowest clicks it off, reasonably intuitive, or not much to memorize.) Behind it is the tone control, on the same shaft, little used; you must poke around to find it, otherwise its existence doesn't interfere.
On the right is the tuner knob with a linear scale. Even if you have never used one a twist of the knob makes the pointer on the center scale move. Reasonably intuitive at that point. If the radio is on, and there are stations, turning this knob far enough makes a sound. Feedback.