Our car radio does not do #bluetooth so after fixing up headphone detection in device tree and finishing #phosh's mpris MediaPlayer2 interface i can now feed sound via a 3.5mm audio cable from my l5 to the car radio via #mpd and #mpDris2 . Will show up on a #librem5 near you soon.
@spacemanspiffy currently we just switch outputs (that's all done by #pulseaudio) but on the lockscreen it could also make sense to pause.