Melt margarine in a LARGE cast iron skillet on top of stove. Stir in brown sugar. Place pineapple slices and cherries on top of the brown sugar mixture. Do not follow the directions on the cake mix. Instead, add the reserved pineapple juice to the mix along with 2 eggs and mix thoroughly. Spread the batter over the pineapple mixture in the skillet. Bake at 350 degrees for approixmately 25 minutes.
Use heavy hot pads to remove the skillet from the oven when done. While the cake is still hot, loosen the edge of the skillet with a knife and turn onto a serving plate. Let cool and enjoy!