I recently purchased a 2015 Hyundai Sonata, Android Auto support was one of the many reasons for my choice. I downloaded Android Auto to my Nexus 6 and connected it to my car with a USB cable... but nothing happened.
I expected Android Auto to "just work" from the dealer but I quickly discovered that the audio system required an update before Android Auto would function. The update process was simple but painfully slow. An "Android Auto" icon appeared in the all items menu once the update was complete but the icon appeared to be disabled (gray and did not respond to touch).
After a lot of experimentation and research I discovered a forum post on Android Central about Android Auto on the 2015 Hynundai Sonata with a couple of suggestions for getting it to work:
1. Ensure Android Auto is enabled under the "connections" menu on the Hyundai Sonata.
I had paired my phone to the car with bluetooth immediately after purchase, any time I attempted to open the connections menu I received an error that it couldn't be configured while connected. I could only see the options once I disabled bluetooth on my phone (forcing a disconnect).
2. Enable USB debugging on the Android phone
This was an unexpected requirement but Android Auto started working almost immediately after enabling USB debugging!
The process for enabling USB debugging is documented in the Android Developer Documentation.