It's been a long time since I've looked at this project; I currently have a DAB Radio/Bluetooth board which I tether to my phone and it seems to do the trick for me for now.
That being said, I'd still like something that could be buried in the glove box and a nice little touchscreen would allow control for music.
In another project (my sample pad controller and electric drum module) ibe changed my plan of attack, and it has left me with a M0-based board from dfrobot and a music playing shield, complete with SD card slot.
Combining this little music player with a small Nextion touchscreen I have (uses software serial to communicate) and an esp8266 to allow a connection to my phone and perhaps an FTP server to drop music onto the SD card and that could leave me with a really handy little player.
If I built a small three channel line mixer I'd be able to have this player, my dab radio (which I use for hands-free calling) and a spare aux in for passengers to use.
This could be fun!