I have asked my local Honda dealership (where I bought the car) and googled and have come up with nothing.
All I know is that my car stereo has been playing music from my iPhone just fine for years, and now with Apple Music suddenly it's telling me that the "accessory" is not compatible and that it's "unsupported". And when it does manage to play a song or two, I can no longer skip back and forth between tracks using the stereo controls because of "Up Next". Apparently there's an incompatibility there, too, and that drives me nuts because I cannot safely skip tracks while driving anymore, I have to do it ON my phone, which is illegal in my state (using cell phone at all while driving). The only way I've figured out to get around this is to create a playlist with ALL of my music on it, then if I play the playlist I can skip around between tracks. But it almost doesn't matter, because I can't seem to get through one song before the iPhone and stereo have an incompatibility issue and stop working.
And the car stereo thing isn't the only issue I've had with it. Whenever I try to sync my phone with my laptop, I end up with duplicate playlists and songs, songs becoming unavailable unless I'm connected to wifi and needing to be downloaded again even though I never deleted them, songs I DID delete months ago suddenly reappearing out of nowhere even though I have never used iCloud or anything else that would store them off of my device. It's crazy.
I find it hard to believe that the update/Apple music is not the cause of the problems when the problems started the day I updated. And while I understand it's not Apple's job to make sure their product is compatible with every accessory and device out there, I would have preferred that all of these changes to the music player be optional, that Apple Music be a separate app for people who actually WANT the streaming music (which I don't). It has completely changed the way I use my iPhone for listening to music, and I was perfectly happy with how it was before. 😟