one idea that will solve alot of problems :
Install in your car a Parrot MKi 9200 kit.
Why ? A lot of reasons:
1- it will connect your tablet through bluetooth , not by wires !!! (optional)
2-you will fully control the music (volume, back, forward , stop, pause) from the included remote control
3- it will also integrate your phone into the sistem, separately from the tablet. You will use voice commands for fast dialing. Also it has build in special processor in order to handle the voice best.
I already have this kit integrated in my car but i use it only for playing songs from my phone , HTC Wildfire S.
The sound is far better then the one of the OEM navigation I have on the car.
Can u send me the link or brand that you use for the stereo facia for the dash?