If you downloaded Ultrasonic from the Google Play Store, it should show up in Android Auto as an available application, and you can start using it for playback without any problems. In this case if you find any issues, please see if it has been reported previously in the Ultrasonic Issues page, or create a new issue if you can't find anything similar.
However, Android Auto won't recognise Ultrasonic if you didn't use the Play Store for installing it. If you've downloaded Ultrasonic from F-Droid, from the releases here on Github, or built one from the source codes, the Developer Mode and Unknown Sources must be enabled in Android Auto for Ultrasonic to be visible.
Enabling Developer Mode in Android Auto
To enable the Developer Mode, you must first start the Android Auto application on your phone, and go to its Settings page.
There are a few different ways to do this:
- On Android 9 or lower: start the Android Auto app, tap the Menu, then tap Settings
- On Android 10 or higher: On the device tap Settings > Apps & notifications (or just "Apps") > See all apps > Android Auto > Advanced > Additional settings in the app (on some phones, this menu is called "In-app notification settings")
After the Android Auto application's Settings page is displayed, scroll to the bottom of the screen, and tap the "Version" section 10 times. The Allow Developer Settings dialog will appear, and you can enable it.
When the Developer Mode is enabled, you can find it in the top right ("...") menu of the Android Auto Settings page.
In the Developer Settings you can enable "Unknown Sources" too.
With the Developer Mode and Unknown Sources enabled, Ultrasonic should show up in Android Auto.