As far as wiring goes, I'm assuming you all have seen these:
Mini Box 12v PSU
That PSU, assuming 80% efficiency, would draw 13 amps...only slightly more than my nav deck, and they have bigger PSU's.
All of their PSU's have three wire hookup, 12V constant, 12V switched or ignition, and ground; with a delayed remote turn on. Not difficult wiring at all, and assuming your whole stereo system is running off an amplifier you can either add a DAC with RCA outs or use 3.5 to RCA adapter to run to the amplifier.
While I wouldn't buy one of their pre-built pc's, you can get better CPU's and Mobo's with the same power requirements and thermal dissapation needs, I would consider their PSU and their mini-itx cases.
Speaking of thermal dissapation, what are the thoughts on computer placement? There is probably room behind the monitor, but the lack of air circulation and the proximity to the heater vents concerns me. Maybe mod the tilt out cubby behind the passenger seat?
Testing will tell for certain, but I'm hoping there is a hardware guy lurking that could answer that...or someone that has done something similar in the past.
Either way I love the way it looks and how it's going.