The devs are currently working on Android Auto support! They are testing it and it should be released in a few releases I can't find the right the link. But they also did announce it on twitter: https://twitter.com/GrapheneOS/status/1721263825192726737?s=19

I shared the literal quote from the grapheneos devs with a buddy, but didn't save the url: "Android Auto support for sandboxed Google Play has been implemented in a feature branch and is being tested"

You can find multiple Quotes by the devs in their discord / matrix channels, just search for Android auto Last reply was an hour ago saying support is coming "very soon"