much more, cause this would work just fine if the load (carputer in this case) didn't take any current.
If you have 12 volts, and then two 10k resistors, put them in series, you have 6 volts between them right?
What if you put a 10k resistor in parallel with the bottom resistor, you'll only have 4 volts on the middle node.
That last 10k resistor represents your carputer, or anything that sucks up current, which obviously a computer will.
Maybe you're thinking of another way of doing a voltage divider, but I know that a resistor divider won't work 'cause you need the same amount of current to go through all of the resistors to get the expected voltage on the output.
You could use active devices (fets, bjt's, etc), but to get a really good design, you end up pretty much building a power supply from scratch.
Hope this helps.