I literally just installed mine and to my surprise it went to Amazon Music. This is what I found on Garmin's website...
"Third-party media apps (e.g. Google Play, iTunes, Spotify) can be used in unison with the device. The Garmin Speak or Garmin Speak Plus will automatically adjust the music volume when it gives navigation prompts, but it cannot control media played through third-party apps; for example, if iTunes is in use, Alexa will not respond to commands to skip or replay songs."
I didn't see the "are expected to be added as available" when they were talking about Spotify. Now I have to tear my car apart again and pull it out to return it. Spotify is 10x better than Amazon music.
This seems to be the case with anything that has to do with car products for Alexa. I have put myself in twice now to get the auto version Amazon Alexa invitation and have gotten nothing since they announced it last October. Beyond frustrated. I feel your frustration.