iPhone 2g to Car Audio - Adapter Question

Hi-

I have an iPhone 2g and a JVC KD-G720 car stereo with a USB port. So far I have not had much luck trying to plug my iPhone directly into the USB port - the screen on the stereo displays an error. So I had a brilliant (or possibly brilliant) thought; what if I could find a USB to 3.5mm adapter, then plug an audio cable from my iPhone to the adapter. The end result of this would be me listening to music over my car speakers...though I'm not certain the stereo would recognize the adapter as a music source...

Anyone know if this will work? Or is there something else I could do? I do have an FM transmitter but am only mildly impressed with the results.

Thanks for any help!

-S