Funny you should mention this; I posted elsewhere on the thread about my experiences with the topic at hand, but my BMW E46 M3 project was the top viewed project on the mp3car forums for the better part of a decade. I don't have the car anymore, but I still have all the crazy parts in a box somewhere.

I implemented a small frontend in linux; I tried to implement a GUI directly from scratch in SDL+GL so it never really got far. I read yesterday about GM licensing unreal engine to build their in-car UI and thought back to this early work even though it didn't go anywhere at the time. I did have a relatively popular touchscreen-friendly windows app for tuning XM satellite radio using the XM-PCR and later the XM-Direct receivers. It supported AM/FM as well using the "RadioShark," a USB connected AM/FM radio. My software supported buffered playback so you could pause and rewind. On the non-public version, it could automatically encode, tag, and save the songs based on the metadata.

Relevant blast from the past:

https://aaroncake.net/projects/mp3player.htm