It will most likely pass audio to the stereo. It may or may not charge the iPhone depending on the pin that the power is routed. Early dock connectors and some current 3rd party connectors use the Firewire 12v pin to pass the charge current. Newer models use the use the USB pin to pass a 5v charge.
You may find that you need this
adapter. It's even sold in Apple Stores (or find it online). $30 or less will do ya.
Here is another
adapter that should also work. A decent explanation of the USB/Firewire issue is on the webpage as well.
My older iPod auto
interface will pass music to the stereo, but not charge the iPhone. Now it works just fine w/the passPORT.
Best of Luck
