Even though my prototype was working quite well, there are still some issues. One of them is that the Bluetooth module gives audible pops on pausing and restarting playback. Another one is that I can't communicate to the Arduino if the music is paused or not and therefore can't put this on the head unit display. Partly for these reasons, and partly to learn, I started on my own PCB.

Full log (Maakbaas.com)