First we will open the Xbox and get comfortable with what lies inside. To see how to open the Xbox LLama.com or Xboxscene have excellent tutorials. After opened use Xboxscene
to decipher which version the box is. If you have V1.0 to 1.5 you can use Sickmods.net's atx to xbox psu converter.
I unfortunately did not have version 1.0 to 1.5 so i do not know exactly how to but you can use the converter to take the power from the car (12v) and use it to power the xbox directly. I had to use an old fashion inverter, which isn't bad but it would of made the project so much cleaner, more built in, if you will.
Well this is what my boxed looks like now. Pretty simple stuff. The bottom left is the two inputs for the 4 controller ports. Top right (bigger box) is the Ide cable which I kept as the flat ribbon style. I definitely think it is possible to get the rounded type of Ide cable for this project but I have not tried it. The small top right box is the power to the dvd drive, incredibly small wires. The mid right is the fan which I thought I should keep in there since it will be under a hot seat with not a whole lot of ventilation. The big middle box is the power cables to the hard drive, pretty self explanatory.