Its possible a Fiat retrofit might be a single part and just a quick change of permissions in the ECU, but maybe not. Hopefully you can find out can then take the simpler & cheaper option of retrofit vs aftermarket.
If you can DIY (its fiddly and not for a total novice, but not pro level stuff) you can get a full kit by company Lindgaard Pedersen for about £200 with stalk, control unit etc. One UK supplier is synergy-automotive.com, based in Chippenham, Wiltshire
I'm happy with my system, looks fairly OEM (family members don't believe it isn't, but I notice the colour shade of plastic on the cruise stalk is different to the indicator stalk!)