On my iPhone4, I used DisplayOut and Resupported. The latter adjusted the aspect ratio to fit any screen fullscreen.
I also hacked my Infiniti's Nav dash and installed a physical switch which allows GPS controls while driving. It also allowed whatever was on my iPhone to display on the Nav screen. My wife would use the screen on long drives.
The RCAto30pin cable cost $5 from DX.
I'm hoping too DisplayOut works again for iPhone5...