I used to want to install a car PC in one of my cars, but nowadays, I've got much less use for it.
In my '13, I've got a Lockpick installed, which has an AV input. I have an 30-pin to composite video cable connected to it, and I've got an old iPhone 4S plugged into it.
I have an unlimited data plan on my real phone, and turn on tethering when I need it. The iPhone 4S connects to the web through it so it has access to Netflix, YouTube, my entire music library in the cloud, any video on my NAS at home, or whatever else I feel like sending to the screen.
Of course, the touchscreen doesn't interface to the phone, but it's not too much of a hassle to just pick the phone up. It's also limited to composite video, but the built-in screen is only 640x480, anyway. Safari doesn't look so great, but any video playing looks fine for being in a car.
