Change default music app so my car plays automatically from Spotify

Hi, I recently installed a new Kenwood bluetooth stereo into my car, and I love the fact that when I get in the car it automatically picks up from where I left off in my songs. Only problem is, it plays from the Music app instead of Spotify, and being someone who uses Spotify pretty much constantly, this is a huge pain. The only time this doesn't happen is when I turn my car off and turn it back on less than an hour later (like when going into a store or something). All the other times, even though Spotify is paused in the middle of a song from the last time I was driving, the Music app will select a random song from my library and start playing that instead. I am out of ideas on how to fix this, as I've already tried researching the problem and done the few things that were suggested, such as completely closing the Music app (which didn't work).

Thanks in advance for any ideas you have to offer!