wandered on to this thread, i have had my acer a100 installed in my dash for about 2 months now
(new registration to this site it wont allow me to link pictures or my build thread sorry)
I dont have a way to power on the tablet without taking the fascia off
it rarely completely dies over night though. I have it set so that screen remains on when powered from stock settings and the car adapter is powered from the accessories position of the key switch sleep set to 15 seconds for when key is turned off. using car mode keeps the screen full illumination while in use.
originally I had used automateit app to do all this but then learned I didn't need it and have since used the above scenario.
Since going to Android for a head unit I can never go back to a "standard" car stereo