before going all out and tearing up a cord, take a look at this: http://cablejive.com/chargeconverter.html
This is made to correct the problems with going from the firewire charging method to the USB2.0 that the 3G uses.
You would not wire a usb cord direct to a cars voltage system. You have to regulate the dc from 12v to whatever voltage the iPhone charges at which is......5v 1 amp it looks like. If they don't market a direct wire in you would be best to "hack" a cigarette lighter plug in charger. I will see if I can find a good option for you and return, hopefully with some good news.
Like everyone says, you cannot charge/power an iPhone off of a direct 12v source (bad things will happen). What exactly are you trying to achieve? Just want to be able to charge and have audio by just using the dock connector? What kind of aux input do you have on you head unit?