Asus_

I have this done also, here were my solutions that most of you may find much better..

First issue was, keeping the tablet alive when the car was off without charge. This was not too difficult. I gutted the tablet of all bloatware and disabled everything that i was not going to use.. limited background applications & processes and also the battery stays in its power saving mode too. I lost 7-10% battery in about 3 days of no car use, so that is pretty damn good.

However, sound quality for AUX was not going to cut it so i opted to hide a single din (no cd port) head unit tucked away in the back there zip tied and plugged my tablet using USB. Done. Quality is a TON better & in terms of power, i ended up jailbreaking the tablet so it turns on when the car turns on and turns off using an app called MacroDroid, where you can program actions based on events, for ex i have it set to 5 seconds after the screen times out and there is no active charge detected (car is off) the tablet will power off. I've also got it to action on when it detects my wifi at home, it automatically lowers the music volume. Great app.

Only con is you got to wait for it to start each time, thats all but i dont have to touch it anymore or worry about it, it now feels like a proper integration

If you do go the head unit route, make sure its cd-less.. they are tiny if you want the smallest and most importantly, make the the head unit doesn't lock the use of the tablet during USB connection which then tries to force you to do everything using the head unit. Some do this, i think the newer head units do this, not sure if its a feature like apple car play or the unit itself but the older head units seem to be more consistent for this