Cramming a miniITX motherboard in a space to fit in your dash isn't as easy as it sounds.
It will require a lot of fabrication, and even then may not do what you want. That fabrication of brackets and mounts is what leads people to locate the PC somewhere else (trunk, under seat, center console, etc.).
The biggest problem in most vehicles is depth. Remember that the motherboard needs another couple inches off the back end to allow for cables plugged into it.