Preheat the oven to 350. Mix together the red velvet cake mix and the 12 ounces of Diet Dr. Pepper. Spray cupcake liners, and fill each liner. Bake for about 22 minutes. Be sure to keep an eye on them. Be sure to complete the toothpick test. Let them cool, and you can put icing on them if you want to. Also, you could sprinkle some powdered sugar on top too.