In a large saucepan over medium-high heat, combine the corn, red bell pepper, green bell pepper, onion, garlic, vinegar, celery seed, mustard seed, sugar, and 1/2 teaspoon salt. Bring to a boil (the pan will contain only a small amount of liquid), then reduce heat to low and simmer 5 minutes. Remove from heat.
Let the relish cool 1 hour before serving. Serve as a condiment with hot dogs and hamburgers, or with grilled steak or chicken. (The relish may be stored in an airtight container in the refrigerator for up to 4 weeks.)