Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Sift together 2 cups flour, baking soda, cinnamon and salt. Set aside.
In a large bowl, cream 2 cups sugar and 1 cup margarine until light and fluffy. Beat in the eggs one at a time, then stir in 2 teaspoons vanilla, 1/2 teaspoon butter flavoring and 2 tablespoons water. Beat in the flour mixture, stirring just until incorporated. Finally, stir in chopped apples and pecans.
Pour batter into prepared 9x13 inch pan. Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.
To make the Rum Sauce: In a small saucepan, combine 1 cup water and 1/2 cup margarine. Bring to a boil. Mix 1 cup sugar, 2 teaspoons flour and 1/8 teaspoon salt and stir into the saucepan. Bring sauce to a boil, stir in 2 teaspoons vanilla, 1/4 teaspoon butter flavoring and 2 teaspoons rum flavoring. Pour sauce over the cake and serve.