I'm actually trying to build something similar, my research brought me to this post.
For me the operating system is second, first thing is to get a functional boot.
The "easy" stuff is:
2. Pi os
3. Power supply to the car. I've seen several people pull it off with surge protectors.
4. Audio output, there needs to be an AMP or DAB hooked onto the Pi either by usb or one of those click on things. But the cables are fairly standard and should be relatively easy to hook up.
There are some fundamental problems that I haven't been able to figure out.
1. Default OEM Car Stereo input. These include the stereo speakers, a wire indicating power on, engine on, car radio cables, and the actual power cable fitted in a 16pin connector (usually). These need to be used to handle correct on/off switching. This can be avoided I believe by making your Pi battery powered and having to manually turn it off/on. Ofcourse that would be less ideal.
2. Radio antenna. I watched a video on car stereo radio's and the guy seemed to imply that there are two radio antennas which receive power from the car stereo. https://www.youtube.com/watch?v=MM24E_X4qNs
. I believe this too can be bypassed by using something like this http://www.adafruit.com/product/1497
. Again less than ideal, is it possible to power the antenna?
3. Actual power consumption, say we have a Pi installed, powered, and hooked up with GPS, Bluetooth, external usb ports, touchscreen, the whole works. Can the Pi handle this?
Then there are some "luxery" problems.
1. My car has buttons on the steering wheel. Is there a way to connect these to the Pi gpio or some other method? If it's not possible or too complex, I would dissassemble the buttons. If GPIO is possible then that limits the use of a click on DAB/AMP in favor of a usb amp.
2. Buttons, ideally I would like volume buttons (preferably on the LCD itself). I think it would be useful while driving to be able to turn down the volume without having to use touch or your phone. A remote control would be fine I guess, as it would be possible to use it without looking at it.
There is another way to go about this. I haven't been able to find one, but an actual car computer, that has the LCD and radio. But which also has an extra HDMI port. You could connect the Pi via aux and have it switch to the other video output with a button click.
Let me know what you think and if you've made any progress.