Is the Aux input a mini-jack, like an earphone plug? If so, all you need to do is pick up the mini to mini plug wire at an electronics store. It hooks up through the earphone jack on your MP3 player and you just turn the stereo to "aux" and control everything through the device. If that's the hook up you've got you might not be able to do it with a phone that holds music though, they often have a unique plug so only their headset will work with it.
If the aux input is RCA's (these would be located on the back of the stereo and would be uncommon for a factory deck) it's easy to find an ipod to RCA adapter.
If all the car has for an input is a mini-jack then there's unfortunately no adapter to get it to work with the USB output. The good news is that it is relatively inexpensive to upgrade the stereo to one that will accept USB input, you'll get an increase in sound quality and you'll be able to control the ipod through your stereo. Shouldn't run more than $200 installed for a decent radio.