Fin?
As in a stock spoiler and you want to replace it with a ducktail?
Becareful when you take it off. Remove the nuts from the bottom of the spoiler inside your trunk. Then get a long, sharp razor blad and keep tracing around the bottom of where the spoiler mounts (between your decklid and your spoiler) - you have to cut off the 3m tape that is used to mount the spoiler.