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From inside the truck with the tailgate and window up remove the panel on the tailgate and on the back side of the lock is a clamp that looks like a "y" (kinda) remove it by just pulling it to one side and pop out the lock and reverse for the new one. You may have to remove the little arm from the old one and install it on the new one.
I just remembered if you have the metal trim on the ouside of the tailgate you are going to have to remove that