The answer is, it depends on which molding your referring to. If it's the one on the edge of the air dam, just remove the air dam and stick the molding back on where it goes (you could use silicone or some epoxy to glue back in place if you have to, but that's usually not needed) and put the air dam back on the truck. If it's the paint protector strip on the truck where the air dam sits on the paint, it's probably best to remove the old one and get a new one, although those don't usually come off very easily. My guess is it's the one on the edge of the air dam?
-G-