I thought about doing this a few years ago, if anyone does want to do this I'd recommend using Tasker, it automates the tablet, so you can get it to load your own scene to make it easier to navigate thru your apps and music player, sat nav etc.
But if the power, on/off button is on the side then it's impossible to turn on/off without removing the tablet, or are we supposed to leave it on standby mode?
It's a nice thought, I'd like to have the added bonus of Google maps, hands free calling through car speakers, but would I need to have my tethering turned on to do this? Have to weigh up all the pros and cons, I'm thinking might be more cons though, need a power pack thing to give power to the speakers, I don't like the touch screen element and I'd like to still be able to use my steering wheel controls for my radio. Tablets have built in battery and I've had some batteries swell up, I'm thinking could it be a fire risk of it always being in the car especially in the hot months, hope they don't explode in the heat.
If there's a decent one with buttons and plugs for steering wheel that's not laggy, that's android and I can install my own apps that's not over £200 then I'd definitely be interested in it. I'm not willing to pay £400 for one as its not worth it in my opinion.
This is my first car that I haven't had to replace my radio yet as I'm able to plug my USB into it, it just hasn't got Bluetooth or hands free calling that my other one had.