Google has just announced Android Auto, a new way to bring Android to the in-dash displays in new cars, and tied with that announcement is an SDK for creating Android Auto apps. The SDK, which will be available soon and tied into the Android L release, will enable developers to create apps for both messaging and music in the car. Messaging apps will work completely with audio prompts and voice control, the same as Android Wear.
Android Auto is designed to simply work with apps currently on your phone, so developers won't actually have to change much in their current apps to have them available to those using Android in the car. Several content partners are already on board with Android Auto, and now the SDK will open that up to all developers.