From the Wiki list of Fiat Group factories it looks like only Ferrari & Maserati production would be affected, as these have facilities in Modena & Maranello.
The other manufacturing facilities appear to be away from the affected area & many Fiat's including the 500, 500c, Panda Classic, Qubo & Doblo are built outside of Italy.
[ame]http://en.wikipedia.org/wiki/List_of_Fiat_Group_assembly_sites[/ame]
From the photo's i've seen online the factory that collapsed looks like a small local factory. Given that Italy is well-prepared for earthquakes i can't imagine Fiat's factories would've been built without adequate resilience, especially those that produce high-value models like Ferrari & Maserati. The biggest potential interuption to vehicle production would be from smaller parts suppliers that might be based in the affected area & might've suffered from damage to their facilities.