Hi David, welcome to the forum. There has been quite a lot of discussion in the 500 forum regarding roof racks/boot racks for the 500 although I would imagine that the options for a 500c would be less than for a tintop.
I have included a link below to one topic on this matter but if you use the search facility tab I believe that there are a couple more discussions including photos that have taken place.
I work at a Fiat dealership and I just mounted the Thule Fiat 500 rack that is "not for a Cabrio" on my Cabrio no problem. I used a box cutter and shaved a little corner of the rubber feet so they would not contact the cloth roof at all. Wind noise is a little loud, but it works great. The top of course cannot be opened while its on as the front of the top is the widest part and would catch as it came past the feet. The design of the feet would prevent this rack from being installed on a glass roofed version.
Hi david i have a 500c ended up fitting a towbar and buying a thule bike rack that fits on the towbar ball, i can get 2 bikes on the back and also have the top fully open. The bike rack was £60 from halfords, the towbar was fitted and wired up by the dealer they had it done by a mobile towbar fitter for £200 hope this helps