Preheat the Oven and Prepare the Monster Cake Pans - Preheat the oven to 350 degrees Farenheit. Grease the 3D cake pan and 13 x 9-inch cake pan with cooking spray.
Prepare the Cake Mix - In a large mixing bowl, combine one box of the cake mix, three eggs, vegetable oil and water. Using an electric mixer, beat the cake mix at medium speed for two minutes. Scrape down the sides of the bowl as needed with a rubber spatula to ensure the cake mixture fully combines.
In a separate bowl, prepare the other box of cake mix just the same as you did the first one. If desired, you may make one white cake and one chocolate cake, or any flavor combination that you would like.
Bake the Cakes - Pour one bowl of cake batter into the 3D monster cake pan and pour the other bowl into the 13 x 9-inch cake pan. Bake the 13 x 9-inch cake pan for 29-34 minutes and the 3D monster cake for 40-45 minutes. Allow both the cakes to cool on a wire rack.
Assemble the Cakes - Once cool, frost the 13 x 9-inch cake with chocolate frosting. Using an offset spatula, create swirls to give the frosting an appearance of mud. Carefully place the truck on top of the frosted cake.
Use some frosting to attach the wafers to the bumper of the car and then mini marshmallows for lights. You can also use melting chocolate in place of frosting.
Decorate the Truck - Color the vanilla frosting with your choice of food coloring. Use the piping bag with a piping tip, decorate the remainder of the car. Frost the wafer cookies and mini marshmallows with chocolate frosting or your choice of color.
On the sheet cake, sprinkle brown sugar on the chocolate to replicate dirt. Cut your lollipop sticks in half and use them to attach the miniature chocolate donuts to the car for wheels. If desired, pipe 'happy birthday' on the cake and add any additional decorations you would like.