Asus_

I have the brainiac mod.

I installed a tiny single din headunit back there and plugged the tablet via usb, so when the car is on, the tablet is getting charged and sound quality is exactly where i wanted it.
If you do go the headunit route, make sure the one you pick allows you to control the tablet. I went in-store and plugged the headunit to a few units that were on display.. Allot of the newer headunits out there with all that fancy android auto and whatnot sometimes takes away the tablet control and forces you to use the headunit. i can get the name of the headunit i used if you'd like... was like $90 and its very very small.

I uninstalled and disabled all apps/programs/features on the tablet that i would not be using. Set brightness and screen response timer/dim etc.. accordingly for car setting... turned off network since i dont care for internet and put tablet to airplane mode. This make the tablet use little power that you'll never really worry about it. I've left the car parked for several weeks and tablet was still good. The tablet has never died on me, its never gone below 95% and i drive only weekends