Preheat oven to 350°. Cook zucchini in a small amount of boiling water until tender; drain. In saucepan, cook carrot and onion in 4 tablespoons margarine until tender. Remove from heat. Stir in 1½ cups bread stuffing cubes, soup and sour cream. Gently stir in zucchini. Turn in 1½ quart casserole. Melt remaining 2 tablespoons margarine. Add remaining stuffing cubes. Toss gently and sprinkle on top of casserole. Bake for 30 to 40 minutes.