I've used, Google's software for connecting your phone to your car, for over a year now. And I can't believe I didn't find the option to change the default text reply that shows up when a message is displayed.
The standard message is, "I'm driving right now." Nothing wrong with that, but it's somewhat impersonal and short, so I've never used it. Not once.
Thanks to How To Geek, I now know the message can be changed!
To change the message, open the Android Auto app on your phone (do not plug it into your car), slide open the three-line menu and select Settings.
Tap the auto-reply option and enter the text you'd rather send. Keep in mind, this auto reply will be used across all messaging apps that work with Android Auto (text messaging, WhatsApp and so on). Finally, tap OK and go back to the main screen of Android Auto and exit.
The next time you receive a message with Android Auto active, your custom reply will be used.