I've got one of these:
http://cgi.ebay.co.uk/CTARNUSB003-R...248?pt=LH_DefaultDomain_3&hash=item2308b44128 (didn't pay that much though!)
Can't recommend them enough. I don't use an ipod, but if you do you can still scroll through 6 playlists (using you 6 cd changer buttons on the standard stereo) on an ipod, but also has AUX input (3.5), SD and USB. I use a 8gb USB pen with loads of songs on it, you can scroll through etc and just play the music on the USB pen (set into 6 folders).
It plugs into the CD changer port in the back of the stereo, and as I use a USB pen I don't need to charge - but can check with my ipod if you want...
I also prefer this as I don't worry about leaving my ipod in the car, potentially a theft risk...