Preheat oven to 425°F Coat a 9 X 13 glass baking dish with oil.
Saute onion with fajita seasoning and oil.
Combine corn, chiles and 1/2 cup sauce in a microwavable-safe bowl. Cover and microwave on high until heated through. About 2 1/2 minutes.
Spread 1/4 cup sauce in the prepared baking dish. Top with an overlapping layer of 9 tortillas. Spread refried beans evenly over the tortillas. Top the beans with the corn and chile mixture, add onions and spread the shrimp evenly, followed by the remaining tortillas. Pour in remaining sauce to cover tortillas. Cover with foil.
Bake the enchiladas until they begin to bubble on the sides, about 30 minutes. Remove the foil; sprinkle cheese on top. Continue baking about 5 minutes or until cheese is melted.