[mook]

You can't show up in Android Auto unless you're distributed through the Play store (third party things like F-Droid and side loading means your app doesn't show in the car).

[Zak]

Android Auto developer settings allow access by "unknown sources".

https://technastic.com/android-auto-developer-settings/

[summm]

This does not work for map applications.

[MobiusHorizons]

Good point, that makes a nontrivial difference for apps like organic maps. I know I have used it for offline navigation over CarPlay several times before.