http://www.scosche.com/products/productID/1667
Scosche Passport Charge Converter. I've used this for about 4 months with my 3G and it also works great with my 3GS. I bought a generic ipod charge+audio to RCA cable from Circuit City before they went out of business and I went home and hooked it up in my pickup and no charge but GREAT audio sound so i bought this and all is well. I'm thinking of getting a 30-pin Dock Extension cable and putting the passport in the dash and just running the extension out as the passport is sort of bulky but it works great for now. It allows all pins to be routed through but on the charging pins it uses resistors to drop the voltage down from 12v firewire to the 5v USB power.