iPhone 3G S doesn't play through kenwood usb input.
I have a kenwood car stereo deck in my car, and it came with a usb input cord. When I plug my sisters nano into it through the iPod cable it works fine. But when I plug in the iPhone it says NA DEVICE and charges my phone, but does not let me listen to any music that is on it.
There are lots of decks out there that are iPod compatible but not iPhone compatible. Google the model number of the deck and iPhone and see what others are saying.