Have you got any apps on the tablet to automate tasks? Say when you start the car the tablet auto connects to the headunit, volume levels set and say launches a music app.
Atm im using the Trigger app (Its free so sometimes its glitchy)... I have set it to auto connect bluetooth to the headunit whenever I turn on the car... Screen stays off (which I wanted) and when I want it to turn on simple double swipe over the proximity sensor and screen turns on. Screen is set to never timeout until it loses connection to the headunit which is set to 15sec...
I dont have it auto launching anything atm... Im still utilising the headunit for radio so I dont see the point... A simple double press of the source button on wheel remote to access bluetooth on the headunit and it will automatically start playing music from the tablet.
I have connected it to a 3000mah power bank which is connected via usb charger for constant charging whenever the car is not on...