Today, WhatsApp introduced a new app update that adds compatibility with Carplay. The additional functionality was first noted on the Dutch site iCulture, and it's automatically available who those who update to version 2.18.20 of the app.
WhatsApp users who have a vehicle that is CarPlay enabled can now see unread messages, ask Siri to read them messages and use the voice assistant to send new messages. Users will also receive notifications of new messages with the name of the sender. The ability to scroll through a list of messages is not available, however.
It's worth noting that this is the first third-party messaging app to work with CarPlay, but its functionality is pretty limited. Apple puts heavy restrictions in place on what developers can and can't do with CarPlay apps, so until it allows more flexibility, this is likely as good as it will get. WhatsApp has been available in Android Auto for some time; with this new development, hopefully we will see more messaging apps on CarPlay.