You might want to consider an inverter - it plugs in to the 12v socket and converts the car's DC power to AC that your computer and other devices can use. Then you don't need an adapter that will only work for your iPod. ThinkGeek sells one that looks like a coffee mug and so fits very nicely in a cup holder. I've had it for a couple of years, and it works well for Apple and other laptops, as well as other electronic products, AND it has a USB plug, so you can charge your Ipod that way as well.
Find it here. They're having a sale right now, so you can get it for about $25 delivered in the US. And no, I'm not affiliated in any way.