I'm not sure what auxillary "power" is, but auxillary input only plays the player of an iPhone/iPod. To charge you need to plug into the "cigarette lighter", 12v source with the appropriate cord.
You can charge it by using auxillary power or you can plug the USB connector into the USB input in your glovebox under your armrest and you will be able to access the music as well as charge your iPhone. Belkin, Griffin, Kensington, etc. make aftermarket USB cables that you can buy at Fry's Electronics or BestBuy for $15-20. You will get a message on your iPhone that says this accessory is not made to work with iPhone. Ignore it and it goes away after a few seconds and your iPhone will charge and you can access your music.
