Combine brown sugar, margarine and corn syrup in a saucepan.
Cook over medium heat, stirring constantly, for 5 minutes, or until mixture is bubbly.
Pour syrup mixture evenly into a greased 9-by-13-inch baking dish.
Arrange bread slices over syrup mixture.
Combine eggs, milk, vanilla and salt; stir well.
Slowly pour over bread slices.
Cover and chill for at least 8 hours.
When ready to bake, combine the sugar and cinnamon, and sprinkle evenly over the soaked bread.
Drizzle with the remaining margarine.
Bake the French toast, uncovered, at 350 degrees for 45 to 50 minutes, or until brown and bubbly.
Serve immediately.

Note: If you don't eat the French toast hot, or if you have leftovers, you should turn the pan upside down onto another platter and remove the contents from the baking pan. This is because the caramel will harden and stick to the pan as it cools, and it will be a mess to remove.