When you build your car PC, your first decisions have a bearing on everything else which comes after. You need to decide where you are going to locate the PC and any input devices. Typical locations for the PC are:-
Personally, I always try to locate the carputer somewhere within the passenger compartment to minimise on the amount of cabling required.
The smaller the installation location, the more important it becomes to select small, low heat emitting components when you build your PC.
Browse through the hardware section for a guide to the various components and how to build a carputer. Do you need help with deciding on the constituent parts of your system?
You also need to decide where you want to position your display screen. It is possible to get roof mounted or sun visor mounted displays, but these are not ideal except when reversing.
A touchscreen monitor mounted on top of the dashboard is probably the easiest option, but...
Fitting it in the fascia gives the best appearance. However, this may call for fabrication skills to achieve the factory fitted look. The only space available may be where the car stereo is fitted. If you remove the stereo, you may need another amplifier and radio tuner.