Heat oil in a medium saucepan over medium heat, and saute the zucchini and onion 5 to 7 minutes, until onion is tender. Mix in the corn. Stir in the diced tomatoes. Season with garlic powder, salt, and pepper. Cover, and cook 15 minutes, or until zucchini is soft.
Remove the saucepan from heat. Mix in the Monterey Jack cheese and Cheddar cheese. Cover, and let stand until cheeses are melted, about 5 minutes.
