Preheat oven to 350 degrees Fahrenheit. Grease the bottom and sides of a 13" x 9" baking dish. We used a metal dish, but if you use a dark or nonstick dish, preheat your oven to 325 degrees Fahrenheit.
Mix cake batter according to box directions. The box calls for 1-1/4 cups of water. In place of water, sub in the Maraschino cherry juice and Sprite. Measure the cherry juice out, and add the necessary Sprite to ensure you have 1-1/4 cups of liquid to add to the batter. Our cherry juice jar yielded about 1/3 cup, so we added just under 1 cup of Sprite.
Mix batter completely. Pour into pan. Bake for 29-34 minutes. In our test kitchen, the cake was ready at 32 minutes. Cool cake completely.
Once cool, use the end of your wooden spoon to poke holes evenly across the cake's entire surface.
While the cake cools, boil one cup of water. Remove from heat and add cherry Jell-O, stirring until Jell-O is fully dissolved. Add the second cup of Sprite to the Jell-O mixture and mix well.
Pour the liquid Jell-O across the top of the cake, being sure to cover every poked hole. Refrigerate for 6 hours or overnight unfrosted.
Before serving, frost cake with Cool Whip topping. Add Maraschino cherries, and serve immediately.