If your iPhone isn’t working with your Mazda5 the fix might be as simple as adding a song to your Music library. On a Mac I needed to do this by plugging the phone into my computer and choosing an album to sync to my phone. Once I did that everything else I cared about on my phone started to work with the car: Spotify and Overcast. I was ready to replace the stereo in our car before a kind person in the Mazda5 Owner’s Worldwide Facebook group mentioned this as a potential fix.
I believe the car will start auto playing the first track in your library whenever it starts. While I love the Chrono Trigger soundtrack I might change it out for some tracks of pure silence.
To be determined is why this works. The Mazda5 is old enough that when you plug in an iPhone it puts it in “iPod” mode. I’m guessing that Apple sets Music as the default audio app in iOS. If there’s nothing in there the car must not have a way to otherwise query the phone for an audio source. If anyone has a definitive answer I’d love to hear it!