Preheat oven to 350 degrees. Melt margarine and put in a 13 x 9 inch pan. Sprinkle brown sugar evenly over melted margarine. Arrange pineapple and maraschino cherries over top, press gently into margarine and brown sugar. Take reserved pineapple juice, add enough water to it to make 1 1/4 cups. Make cake batter as directed on package, except use pineapple juice mixture instead of the water. Pour batter over pineapple and cherries. Bake 40 to 45 minutes or until a toothpick inserted in the center comes out clean. Immediately turn the pan upside down onto a heatproof plate. Leave pan over cake for one minute. Serve warm or cool.