There is an official tow bar for the new 500 it’s shown on the Italian Fiat website. I looked into getting one fitted and got 3 different prices from 3 different dealers. I contacted Fiat UK and they were not totally sure of the exact cost but it was averaging out to £450 fitted. I have opted for carrying the bikes in the car as after seeing how my dealer fitted the rear number plate I was not going to let them fit a tow bar. All I do is remove wheels, seat and pedals and I get two full size bikes in the back. I looked at the roof bar option as well and the glass roof seems to be causing a problem as the note on the Thule site is that the roof bars they have for the 500 is not suitable for glass roofs. If you can get a definitive answer on the safe working load of the official rear bars from Fiat you might be able to engineer some form of rack. As I said I have opted for carrying them in the car for now. Hope this is of some use.