I really like that tablet idea. You're going to have to skew into Stereo and Electronics and explain how you did that all.
The Nexus 7 does digital audio out the USB port, so using an OTG cable you can charge the tablet and run data/audio in/out at the same time. The short version is the tablet does digital audio out to a DAC (digital to analog converter) and the DAC has RCA outputs that then go to a normal car audio amp. The DAC I'm using does 16 bit/48 kHz (at least as good as CD quality sound, which is very good) sound quality, so it will also sound very good. Using the headphone jack gives pretty crappy sound quality, which I didn't want. The longer version is that I put a USB hub in between, so I will also be able to run an external hard drive, backup camera, or anything else I can plug in to a USB port and the tablet will recognize it at the same time. I'm also pretty anal retentive about people doing them powered off the cigarette lighter, so my USB hub is 12V powered from the factory harness. I'm using a normal PAC aftermarket radio interface (like you use when installing an aftermarket radio to keep all the factory chimes) and powering the hub off the RAP wire and also using it to control factory steering wheel controls (when I swap my steering wheel control column in). So, when you turn the key on it powers the hub and everything is connected to the hub, turn the key off and it turns off like factory when you open the door. Android has an app called Tasker that allows you to write commands for how the tablet acts under certain situations. In this case, when it sees power it will wake the tablet up, when you remove power it puts it to sleep. Then I get badass Google searches, that can go to Maps, Navigation, etc. Plus streaming YouTube, Google Play, Pandora, etc. There is also an app called Tablet Talk where the tablet can be your phone, so I can make calls or text through the tablet. All kinds of neat stuff you can do.
The DAC will feed some older Memphis amps I've been saving, Hertz components in the doors and probably one of Sundown Audios shallow 10" subs. At some point I'll add a DSP (digital sound processor) to really tweak the sound. But overall it should rock pretty hard.
Here's a test video I did of making it all work together. The output is through a little speaker out of the DAC, but in the end that will go to amps instead. Notice nothing is coming out of the tablet speakers for sound or anything plugged in the headphone jack of the tablet.
https://youtu.be/70Jd5EnPjtA