Preheat oven to 325 degrees F. Spray two 9 inch cake pans with cooking spray.
Add the cake mix to a large mixing bowl and pour in the Sprite Zero.
Use an electric mixer on medium speed for two minutes.
Cut the cranberries in half then add them to the cake batter. Gently stir in the cranberries.
Add half of the cake batter to each pan. Bake 34-39 minutes or until a toothpick inserted in the middle comes out clean.
Let the cakes cool completely. Add one cake to a cake stand, add half of the Cool Whip and spread evenly over the cake.
Add the second cake and spread the rest of the Cool Whip on the top and sides. Top with fresh cranberries.