There are a TON of options out there.
Some allow you to control your iPod using your radio controls but you will be limited by the number of playlists you can access, which is the down side.
I believe the latest Dension unit allows for multiple playlist access but I forget what the new product is called.
For me I needed the option to connect up more than one external device. I have a portable XM satellite radio unit and an iPod Video. For me, controlling the iPod through the radio was pointless because you get NO visual or audio feedback as to what your scrolling through (major negative). With no feedback you need to be looking at the iPod and if you need to be looking at it, then why not reach over and control it manually since it's faster?
Anyway I degress, with needing two inputs I chose the USA-SPEC adaptor, which has two separate sets of RCA inputs which allowed me to connect up my two external audio devices. It works great and was 100% plug and play with the Audi Bose Symphony II radio.
Check out the links at the bottom of Apple's page for aftermarket solutions.
http://www.apple.com/uk/ipodnano/carintegration.html
For the most part they all do the same thing with the same audio quality. I think the Dension product has the best benefits since it can be bought with a a nice cradle to dock your iPod.
I've personally owned the Blitzsafe adaptor and it sounded fine.
