The problem: the original mp3 player was a $15 Sandisk Sansa C250.  There's a video of it playing a video, when the buttons were still intact.

Through heroic soldering efforts, it lasted 10 years as the mid day heat & SD card failures destroyed it.  What was left of it finally got stolen.  

The mane features were the ability to start on ignition, stop on stop & advance playlists automatically.  No commercial solution emerged to replace it, for such a low price.  Everyone else just got new cars & put up with being required to advance through the playlists manually.  

The problem with any custom solution has been creating a user interface for the raspberry pi that doesn't get stolen & the fact that lions hardly ever drove in the last 7 years.  With the raspberry pi W & the Generalpro, there was finally an easy & cheap way to do it.  There could be a GUI on a phone & a physical interface could be reduced to simply skip.  Never really used anything but skip.  If a really bad playlist came on, it wouldn't be unthinkable to use the phone browser.

The devil is in the details: 

Should the GUI be an app over bluetooth or a browser over wifi?

Should it be installed inside the car radio after tearing it open or stuffed below?

Should the GUI do anything besides navigating files & deleting songs?

Should it just continuously play back or have a notion of pausing?

Should it compress dynamic range in realtime?

Should it implement EQ?