I have this head unit for an AUDI and here are some things I found:
The radio and other UI components come from the system app called "MainUI" which gets modified depending on the company/seller who puts their own touches to it. It is signed using the AOSP android platform certificate and you can install/replace it with another MainUI and things mostly work. MainUI is also moddable if you know how to recompile the apk as the code is not secured in any way. Simply resign it with the platform certificate and install via adb/usb.
The unit boots up in two different ways depending on how long it has been left off.
* cold bootup which starts with the animated "android" splash- this bootup will allow apps installed to run if it has the BOOT_COMPLETED intent in its manifest.
* Quickboot- this is where the bootup is fast and bypasses normal android boot. In order to have an app start in this bootup mode, you need to set the system property, for example, to start the "navigation" as per settings:
setprop sys.qb.startapp_onresume "L#P com.ts.MainUI#A com.ts.main.navi.NaviMainActivity#T 2#I 5"
the components are seperated by "#", and the format is L#P <package>#A <activity>
then you can add optional T <number of times run>#I <number of seconds between each run>
In this example it will try to start the navigation activity twice and 5 seconds in between. I noticed that if you just try to run it once, sometimes the UI is not ready and the activity fails. The "L" at the beginning means to try to rerun apps that were already running before shutdown, it only works sometimes.
Sometimes the system properties will not stick and gets reset by the device. I do not know why this is the case. Therefore to make this more permanent change, it needs to be added to the /system/etc/prop.default file.
Changing any system files requires booting into recovery (via android developer options and adb reboot recovery), mounting the system partition and using the root access "su":
then mounting the system_root as rewritable
mount -o rw,remount /system_root
Someone else posted the above information in another thread and was also on the russian 4pda forums, but ill add it here as well.
Bluetooth tethering works well generally, although you will find sometimes it will disconnect by itself. In order to fix this, go to the "phone" app and reconnect from there.