iPod Dock Charge Adapter
The Charging Adapter from Scosche should work for you. I used one of these in a car that had a iPod dock for the radio, but used the older Firewire pinout, and therefore would not work well with a newer iPhone. 

This one looks slightly different than the one I had, but it should be the same thing. Griffin I think also makes a similar product. Its a common problem for car radios, so maybe searching for that will help find similar products. 

Update: The Scosche Passport is what we actually have. This worked great connecting a newer nano, iPhone 4 and 4S to a Firewire charging deck.