This apparently has occurred for a while. For example, the iPhone 4S:
https://discussions.apple.com/thread/3518697?start=0&tstart=0
Is it always playing the same song when you turn on the car? The first song? That is most likely your car's fault. Volkswagen (in the link) had a update to disable this.
You could make a song with silence and name it "aaaaaa" so it shows as the first song. Then manually stop the music each time.
Considering this is such an old issue, its likely the car's fault. This thread from 2011 talks about it being a car setting:
https://forums.macrumors.com/threads/any-way-to-turn-off-autoplay-with-bluetooth-audio.1245173/