I've started to do something similar with my Pandigital Novel in my Prius.
The way I see it, you have a few options. The first would to be to just get an auxiliary input for your current radio and a car charger for the g tablet. Then you could mount it and run the wire how ever you like. If you just simply use double sided tape or velcro like I did in my Prius, this would be the easiest option and would be very reversible.
If you want to get more involved you could look into doing the same thing but moving the stereo somewhere underneath the dash and make a custom mount to put the g tablet in it's place. This would obviously be much more involved.
In both cases, you would just set the radio to the auxiliary input, set it a a higher volume level, and then control the volume and media via the tablet. However, I see the g tablet's limited viewing angle being a huge problem in this setting. Combine that with the very reflective surface and bright day light, you may have a hard actually seeing the device. I would test mount it with some velcro for a day just to get the angle right.
Another thing that I've noticed while working with the Pandigital Novel is that when the power is plugged in, it outputs a very noticeable whine through the auxiliary cable. This may be because I was just testing it with the wall charger plugged into an inverter, I may have better luck with a car charger.
Best of luck, be sure to let us know if you can get the viewing angle right!