Change default music app to Spotify Whenever I use Spotify over bluetooth and try to use my car's controls it, simply pauses the Spotify music and launches the default music player.
The Spotify music plays just fine, however, when there comes input from the Bluetooth, it pauses and starts the default music player.

Spotify is not listed in Settings -> Apps -> Default Apps -> Music, only Google Play Music and the default music player is listed on my Huawei P7.
I've tried using several softwares including Default App Manager, Better Open With, A2DP Volume and Bluetooth Connect and Play - however none work.

It would seem like when I click on the car's input it transmits it to the phone over BT, however, since the default music player is the default player it starts that app instead of Spotify.
You can ask Google Assistant to "change the song". It works perfectly. 
Start a random song on Spotify.
Say "Ok Google" and wait for Assistant to pop up.
Say "Play the next song"
And she does it.
Works like a charm, as long as your phone's microphone works properly.