Dan-cer I thought that with GrapheneOS and a modified Android Auto, everything would work without Google.
My understanding is that the GrapheneOS developers have not modified Android Auto in any way (just as they have not modified the Play Store, Play Services, etc., in any way). Those Google programs are provided with a carefully-tweaked execution environment, so that when they make privileged API calls under the assumption that they are privileged programs, appropriate effects are approximated without them being privileged programs.
Android Auto is a large pile of Google code. Installing it on GrapheneOS is not "without Google". In theory somebody could implement a from-scratch equivalent of Android Auto by invoking the appropriate protocols to speak with a car, but that would be a large undertaking.