i got this "y cable" off of http://www.installer.com/aux/index.html . All you need is an auxilary input, set of red and white cable, in the back of the headunit. Then out comes a headphone jack. Cost me $7.99 and hasnt failed me yet.
i went with the aux input and then the RCA to headphone route. the aux input gives about 3" leads of RCA female connectors then the cable to headphone plug works great for my iPod video. this setup would also work for the nano. and the whole deal cost me $21. the interface for specific brand headunits I would be wary of...take your iPod to best buy and try out the interface on one of their radios before you buy one and see if you like the interface. the controls of the interface for kenwoods at least arn't all that spectacular...
my friend bought the interface for his pioneer and he loves it he was able to get rid of all his cds and only caries the ipod plugs it in the glove box and controls it from the screen he has a 6.5" screen though
JVC makes an interface also. Try like a best buy. around 100 or so. I am sure on their website it will tell you which model is works with. The only thing with those adaptors is sometimes you need to update your firmware in the ipod for it to work correctly