The way to do this is to get an auto AC inverter. Here's one
http://www.amazon.com/Tripp-Lite-PV...id=1386092140&sr=8-6&keywords=car+ac+inverter
I have an Coleman-branded unit that's maybe 6-7 years old that still works fine. It lives in the trunk, rarely comes out, but I know it's there.
There are one-piece inverters that plug directly into the cigarette lighter socket, but I don't recommend these units as they are rather bulky, making them a difficult fit in the tight confines of most autos. Also, there's probably some weight strain on the stem that plugs into the lighter socket, something that can be easily avoided by using a corded model.
Another benefit to using an AC inverter with normal socket(s) is that they are naturally compatible with a wide range of powered devices. A dedicated computer charger won't work with anything else, which makes it a rather poor purchase.
For this latter reason, I refuse to buy any car charger for iPhones with a built-in cord. The dual port USB car chargers are the way to go, with at least one of the ports supporting 2.1A power (for iPads, etc.). Here's one:
http://www.amazon.com/Belkin-2-Port...86092498&sr=8-7&keywords=ipad+usb+car+charger
Again, this can be used for a wide variety of devices that charge via USB. Dedicated iPhone car chargers are useless for other devices.