June 2018 : If you own a ROOTED phone, go to the releases section to download v2.0.
Apr 2018 : Google has blocked all custom apps on Android Auto, hence this app won't work anymore.
youtube-android-auto 
A youtube player for Android Auto. Please visit http://www.thekirankumar.com/blog/2017/12/29/play-youtube-video-android-auto-app/ for downloading the APK.
Credits
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Unofficial Android SDK https://github.com/martoreto/aauto-sdk Without this SDK this project wouldn't exist. Thanks to Martoreto for reverse engineering the Waze app and coming up with a easy to use SDK and a sample app.
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VideoEnabledWebview https://github.com/cprcrack/VideoEnabledWebView This webview makes fullscreen videos easy to implement.