I have tried about everything with the HDMI.
1. Apple digital A/V adapter
2. Apple TV(requires internet for apps, network for airplay)
3. Chromecast(requires network for casting)
4. Mac Mini(requires keyboard and mouse, hotspot for internet)
5. Android mini computer/tv stick(requires wireless keyboard and mouse)
6. Sony mini camcorder(for dashcam)
What i have figured out, is for most of this to work, you have to have to have either internet access and/or a network(via router or hotspot) in your vehicle. The other issue, is hdmi will only play on the head unit, and can't be mirrored to the rear screens(which was my goal to begin with).
I wanted to have youtube, netflix, hbogo, etc, for the kids on headrest monitors, not for viewing on headunit.
i have hdmi splitter in glove box, for easy hookup to any source, also hotspot for internet and network setup.
After all the testing, I am working to root the android mini pc, and only use hdmi for AppRadio(for the DashCommand app). leaving my iphone hooked up to usb1.