SAN FRANCISCO, CA - JULY 24:  The new Google Nexus 7 tablet, made by Asus is displayed during a Google special event at Dogpatch Studios on July 24, 2013 in San Francisco, California.  Google announced a new Asus Nexus 7 tablet and the Chromecast SDK.  (Photo by Justin Sullivan/Getty Images)
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How To Use Any Android Tablet As An Android Auto Car Head Unit
Android Auto allows you to connect your phone to your car’s system and control apps like Maps and Music using the built-in touchscreen head unit. If your vehicle lacks support for the operating system, there is a way to add this feature by transforming an old Android tablet that you might have lying around into a head unit that even works on older cars.
You will need an Android phone to be used as the host device and a larger Android tablet to display the Android Auto interface. Before you mount the tablet, you must download and install the $4.99 Headunit Reloaded Emulator app from the Google Play Store or the developer’s website and then transfer it to the tablet using an SD card or USB stick.
After installing the app, you will be brought to an interface page to initiate the connection between your phone and the tablet. For wired connections:

1. Mount the Android tablet and connect it to your phone using a USB cable.
2. Set the mode to File Transfer/Android Auto. Under the “USB Controlled By” menu, check “This Device” rather than “Connected Device.”

3. Open Headunit Reloaded Emulator on the tablet. Tap OK when asked to grant permission to “Modify System Settings.”
To connect wirelessly:

1. Connect the tablet to your phone’s Wi-Fi hotspot. Open your phone’s Settings > Connections > Android Auto.

2. Tap "Version" several times to enable developer mode. Press the three-dot button and select "Start Head Unit Server." On the Android tablet, go to the Headunit Reloaded Emulator app and tap "Wireless."
The Headunit Reloaded Emulator replicates all of the features found on a head unit and will load up with your default navigation app and media player. You can change many of the default options in the Settings menu, such as your output resolution, the orientation of the Android Auto interface for right-hand drive cars, and toggling between day and night modes.