Heat broiler. Place prepared grapes in a large mixing bowl. Coat grapes with sour cream. Place grape mixture into a 2 quart baking dish. Sprinkle brown sugar over the top. Broil, rotating as necessary, until the sugar is caramelized and crispy in spots, taking care not to scorch it. Chill before serving, at least 1 hour or up to 24 hours. Top with toasted pecans just prior to serving.