Thorned_Rose Thanks for reporting back! This information may well help other users.
Thorned_Rose

- Joined 2 Feb
I found a working answer on Reddit.
Referenced Reddit PostTo permit Android Auto to sideload apps, you need to enable the Developer Settings of Android Auto (not the phone) and allow AA to load apps from Unknown Sources.
Here is a step-by-step guide.
- Setting initial permissions
I believe you already set the permissions right from Settings ➔ Apps ➔ Sandboxed Google Play ➔ Android Auto. - Enabling AA Developer Settings
Settings ➔ Apps ➔ Sandboxed Google Play ➔ Android Auto, on the bottom of the screen click "Version" 10 times. This will enable "Developer Settings". - Enabling "Unknown Sources"
Now you have "Developer Settings" for Android Auto enabled, you can access it from the top right menu (3 Dots). Click on "Developer Settings", scroll down, and find "Unknown Sources". Enable it and restart the phone.
Now you should have access to the missing apps.