c_walsh, they are just held in with clips IIRC. I would take a look at getting the clips from your local Ford (more expensive yes, but you know you are getting the right part) or ordering them from fordparts.com. Or if it's badly damaged, now may be the time to upgrade to GT rocker panel covers, which are painted body colour and look cooler than our black textured plastic ones.
1966 Mustang Sprint 200--Wimbledon White
1998 Mustang V6 Coupe--Silver--RIP, Totalled by brother
2005 Mustang V6 Premium Coupe--Sonic Blue