At the library I noticed the book "Build Your Own Car PC" by Gavin D. J. Hooper. As the title suggests, this gives a step by step guide to building a personal computer into a car for entertainment, navigation and car diagnostics. The book takes a purist approach using a VIA SP Mini Itx motherboard which fits in a case the size of a car radio (DIN slot).
Such a tiny PC requires special low height components, for the memory cards and heat sink, a more rugged disk drive. The book discusses peripherals such as tray less DVD drives, radio and TV tuners, small LCD screens.
But what is not made entirely clear is why you want to do all this. A car PC would make an excellent entertainment system, able to play movies and music. A GPS antenna will turn it into a navigation system. An On Board Diagnostics (ODB) connector will allow the PC to read diagnostic information from the car's engine management system. But the details of how you would use these is not covered in great detail in the book, just how to put them together.