[Paid Software Announcement and Discussion] mikereidis:
Troubleshooting:

Nothing more disheartening than trying a cool new app, and.... it doesn't work. :(

Public Enemy #1: Cheap/Bad USB OTG cables.

I got 3 cheap OTG cables and 2 straight and 2 right angle adapters a few months ago. The cables seemed OK in the beginning and now they're all flaky. The adapters didn't last as long and some were flaky new. 7 cables and adapters and they're all garbage. :(

I've been happy for a few weeks at least with these $10 Y cables that can power/charge both devices. I'll see if they're still good in the next few months: http://www.amazon.com/Micro-Cable-Power-Samsung-AtomicMarket/dp/B009YPYORM

But even with the best cables, your phone and tablet micro-USB ports can be a factor too. Car mounting, with constant vibrations and bumps adds to the challenge.

When I get Android Auto over Wifi working better, I hope USB will just become a backup connection. But wifi has it's own issues too.

Cheap or Chinese devices and hardware HUs are a "crap-shoot": So far I don't think I've had a single success report for Chinese devices. The ROMs and video decoding are often bad or VERY slow and USB is questionable. Some devices do not support USB Host mode or have a bad software implementation.

I recommend Nexus or good quality tablets from Sony or maybe Samsung. All tablets that I own work and are officially supported, including: Nexus 7 2012, Nexus 7 2013, Nexus 9 and Sony Xperia Z2 tablet.

July 24 or later releases recommended for testing: http://xdaforums.com/showpost.php?p=62014837&postcount=519http://xdaforums.com/general/paid-software/android-4-1-headunit-android-auto-t3125252/page39

July 24 has a bottom "Test" button to test H.264 video decoder (some phone/ROM decoders are bad or VERY slow.)

Also has a bottom "SUsb" button for devices with USB Host Mode supporting kernels even without Android support.

Also allows manual USB device selection when automatic doesn't work.

MANY possible fixes:

- You must have 2 devices for the Headunit app, so try reversing them to see if that works. This requires the Android Auto app and Google app, Google Play Music, Google Now and Google Maps to be installed on both devices of course.

- You usually need 2 cables: The micro-USB side of an OTG cable to the Headunit app device/tablet, and the full-size USB connection to a "normal" phone/USB cable connected to the Android Auto device/phone.

At least one person reversed the cables, without reversing HU and AA devices and it worked. USB can be funny; there are MANY possible jumper, resistors, wires etc combinations, especially with specialty high current charging cables.

- With some devices, an OTG Y cable MUST be powered, with others it MUST NOT be powered, and for some it doesn't matter and power can even be stopped and started with no effect.

- "Fast Charge" should be disabled if it's a kernel setting. Saw this with Elemental-X kernel on Nexus 7 2013. This kernel also has a setting to allow charging in USB Host mode.

- Sometimes enabling USB Debugging helps, sometimes disabling, sometimes turning it off, then back on. Do this at both ends: Headunit app device and Android Auto device.

- Unplug and replug cables, at both ends and in between. Ensure they are firmly pushed together. July 11 release allows you to see devices disappearing and re-appearing if cable connections are flaky and moved.

- Before starting Headunit app, plug all cables together. If Camera Importer or Gallery, or other apps pop up, hit Cancel or Back to deal with them first. Then after a few seconds of no more popups, NOW start the Headunit app. Plugging/unplugging while the app is running may work but is less successful than starting the app after plugging.

- Some devices/ROMs have various USB settings to try, such as "Charge Only", MTP, etc.

- Use the Exit button or Back key to terminate the Headunit app and try again. It kills it's own process for an extra fresh start next time.

- Use Recents button to return to the Headunit app if Android Auto covers it.

- Avoid screen switching to Portrait mode for best chances of success. Home-screens that force portrait mode can create problems, as can lock-screens.

- Self or SUsb modes require SU/root access. Sometimes the SU prompt is covered and not visible. In such cases manual SU configuration, or setting SU default to "Grant" can help.