TonimusMaximus, I wouldn't run it right off the car battery without some sort of regulator. What size drive are you using, and what kinda space do you have to work with, I am still a little confused about the specs here. Are you going, sans-casing? What about an external drive kit a dc -> ac converter and the typical power brick stuffed in the glove-box or something?

Either way, I think you are going to need to store something a little bulky in another location and run a wire...
_________________________
"Fix it 'til it Breaks."

Jacob - EiC & Director of Technology
Mac Pro Quad 2.66 - 4GB RAM
160 GB SATA RAID 1 - 650 GB Storage
Quad 19" Widescreen LCDs
Accessorized to the Hilt