tintagel Aurora can't install android auto with an error saying it can't be installed via third party.
That's because you aren't supposed to install it from Aurora, you install it via GrapheneOS preinstalled App Store
See here: https://grapheneos.org/usage#android-auto
To install Android Auto, use the GrapheneOS App Store. Android Auto can't be installed through the Play Store or other app sources. Android Auto depends on sandboxed Google Play, you'll be prompted to install it if it's not already installed.
After installation, Android Auto has to be set up from the Settings > Apps > Sandboxed Google Play > Android Auto configuration screen, which contains permission toggles, links to related configuration screens, configuration tips, and links to optional Android Auto dependencies.