you need it hooked up to "something" it needs at least power, if you want it to play music you need it to go to a amp for the speakers. If you want it to only connect, and not hook to the earphone jack you will need a true port which usually still requires a computer of some sort. IF you are permanently mounting it, there are other options.

if you are going for the cool factory of a tablet.. thats on thing, a car-puter or a touch screen head unit with a 7" screen might do all you want it to do.. and might be more understandable install.

read up on this.. seems like decent info..
Nexus 7 makes the best in-car entertainment system ever | Android Central


i am more apple oriented. but understand/appreciate the capability of android product for this.