Hey, There is an AUX on the Fiat 500 Pop but its internally located behind the stereo it seems. If you were to install a kit to allow an AUX connection you would lose functionality of the CD player.
I have a Pop and the best thing to do is to get an FM transmitter if you just want to listen to music. But spend a premium price for a Belkin one (i.e £30) instead of one for a tenner off ebay.
Hope it helped.