I just bought a 7.0 galaxy plus 3g with this in mind. I got rid of my phone when I bought this tablet too. I spoke on my phone 15 minutes last month and all that was in the car. I can do that with voip...
I intend to do this install on a 2011 wrx. I just bought the JDM radio bezel that is 6.8x3.9 inch cut out (stock usdm is 8" wide). With a bit of dremeling I should be able to slide my tablet directly into the top of the bezel. I plan to put a backing plate so it looks finished with the tab removed. I want removal to be painless. As I plan to use this tablet a lot.
I'll be completely nixing the head unit in my car. To provide power to the speakers I will buy a clarion equalizer and a smallish 300w 4 channel amp probably the nice alpine on amazon for 120 bucks.
Here's where the fun begins! Audio out should be no big deal. I found cables that split the 3.5mm trrs audio into audio out and mic input. So I'll just run rca outs to my clarion eq and run my mic into the device. Done!
My wrx has steering wheel controls. These tie into a stock unit typically. That's no fun. I plan to buy a sparkfun ioio with bluetooth and intercept the SWC signals. Then using the google ADK I can read the pin signals and respond to things like skip track, volume, mode (use mode to switch open apps), pick up, hang up and voice command. I also want to use the ioio to interface with the headlight dimmer and illumination as well.
I just got my tab a few days ago. My bezel should be coming in the mail early next week. Once the bezel is done, I'll rip out the head unit and do the amp and equalizer install. At that time I'll capture the steering wheel control voltages and start hacking on the ioio.
I'll start a project thread for all this once I do the amp install.