It's very easy to do it. All you need to do is build a very simple voltage divider circuit with a couple of resistors.
I built one recently dropped 12v to 0.37v.
Are you looking to run one component on 6volt or the whole car??
Deckie,
Is yours bad? I'd check your brushes first, they are cheap and easy to fix.
Yeah but at what amps? You need whacking great resistors to handle the power from the generator