1. The usual caveat: You have to actually have a head unit that supports Android Auto — not AppRadio or other things like hat.
2. OK, here's how things work. ... Messaging apps won't actually appear in Android Auto as installed apps. That is, you won't see them listed like you will entertainment apps (podcatchers, other radio apps, and music players, etc.)
But you WILL be able to send and receive messages. Whatsapp doesn't handle groups in Android Auto (at least not that I've figured out), but it's easy to send messages to individuals. ... Just say "Send a Whatsapp message to xxxxxxx."
When you receive a message you'll get a notification on the display, and AA will ding at you. You then have the option (or not) to listen to the message. WARNING: If your kids are like mine and only message you in emoji, sit back and relax because it's going to speak every. single. one. at you ...
Hope that helped. Good luck.