Aux is an audio connection, it has nothing to do with USB. Connecting the phone to the USB connector in a car that's expecting an mp3 player, if the phone is set as a media device (pull notifications down) should work. (The music files probably have to be on the root of the SD card.) Connecting the earphone jack to the Aux input of the radio will work.
Your car dealer's service department has probably done this hundreds of times, and will be able to tell you exactly what to do.