[Paid Software Announcement and Discussion] mikereidis:
How do I use the Headunit app?

At this time I can't recommend this for use while driving. It's ONLY for testing within the safety of a home or office... If you can't understand that this Headunit app is still VERY experimental and potentially very distracting while driving, then please don't run it. It's not my fault in any respect or measure if you hit a tree or kill people; it's ALL on you... You assume all responsibility for collisions, injury or the advent of Skynet...

App requires Android 4.1 ICS or higher, but I've mostly tested on Android 4.4 KK, 5.0 and 5.1 Lollipop..

I tested successfully on these, for the Headunit/Tablet side running my app:

Tablets are recommended for a decent size screen in a vehicle:

Xperia Z2 Tablet - CM12 - This 10" screen is wonderful compared to the 7 inch/17 cm 800x480 Pioneer 4100-NEX screen, despite that the video is still only 800x480 at this time.

Nexus 9 - stock - This looks pretty nice too and the CPU power helps make it look good and pretty lag free.

Nexus 7 2013 - stock

Nexus 7 2012 - stock

Phones can also work, but small screens are less safe so I can only recommend these for testing:

HTC One M8 - stock

HTC One M7 - CM12

HTC One XL - CM12

Xperia Z - Lollipop

Xperia Z1 - CM12/FXP

Moto G - CM12

GT-N7100 Note 2 - CM11, but with some video issues.

These phones have major issues or did not work for me:

GT-I9300 GS3 - CM12 unofficial

GT-N7000 Note1 - CM11

GT-I9100 - Android 5.1 ROM

GT-I9000 - Android 5.1 ROM

Note that Headunit/Tablet side issues are largely separate from Phone/Mobile Device side issues. Phones that do not work with Pioneer or other AA head units will not work with this app either, and only Google and the phone OEMs can fix that. This includes MANY very popular Samsung devices. I note that my older Samsung devices also have issues on the Headunit side, and this may be related to general USB issues on Samsungs. (?)

The Headunit/Tablet side running the Headunit app needs to connect to the phone running AA with a USB OTG cable. The OTG cable small micro-USB side needs to plug into the Headunit side.

Many devices can do USB Host for the Headunit side, but can't supply any power to the phone. For these devices a powered USB hub is required, and is recommended for all devices anyway, or the Tablet/Headunit device will quickly lose power to running the phone device.

The phone running AA plugs into the USB hub output side, and should be the only device so plugged in (or the device my app connects to will be random-ish, with HTCs having highest priority.)

When they are all plugged, be patient and wait at least 6 seconds after plugging the USB before starting my app. Most likely the Gallery app will open on the Headunit side. This is annoying, but at least it confirms the connection is good. You may have to wait, then hit Back to exit Gallery. If you get any popups, hit Back or Cancel; you might get several.

Latest AA companion apps to run on the phone side are here: http://www.apkmirror.com/apk/google-inc/android-auto/ This requires Android 5.0 Lollipop+ on the phone side as per Googles' decision, and also requires Google Mobile/Play services to be installed and an account set up.

Now you can start the Headunit. You should be prompted to allow access to USB twice. Select OK both times. Don't bother to check the "Remember" box because it never works and it's a silly Google thing. Better solutions are possible, especially if rooted.

If you have never connected the phone with AA to a headunit before, you will be prompted to do a bunch of stuff on the phone side. If you are outside the US you must manually update to the latest AA app from APKMirror etc. You may have to download Maps and Play Music if you don't have yet, and say Yes or "Standard" etc, to various prompts.

After all this, hopefully you will see video on the Headunit screen. You will likely see a safety warning prompt etc. Read it and understand it, and select OK if you dare.

Hopefully you will now come to the main/home/Overview/Now screen of AA. On some devices, video may be imperfect. Select different functions on the "rail" at bottom to help clear it up.

Touch works, but not Multi-touch yet.

Audio output was through the phone last I looked, but microphone does not work.

If you hit Back while in the app, it should exit and the AA connection is broken.

To open a new AA connection/restart the app you MUST disconnect and reconnect the USB, at any point in the chain.

It generally seems possible to turn the screen off and return to it after powering it back on, but if the orientation changes from landscape this may not work. Same for leaving the app with the Home or Recents button; if it switches to portrait, you can't get the video back and must stop the app, reconnect USB and restart the app.

I'm finding that quite a few of my phones running custom ROMs will not run AA on the phone side properly. In some cases a custom DPI really messes up the video dimensions and you can't even select the startup OK button. I see this on my One M7 with CM12 and many people see the same on other phones w/ custom ROMs, whether using a Pioneer headunit or this app.

Enjoy responsibly. :)