Spray slow cooker with nonstick cooking spray. Place chicken (frozen, thawed or fresh) inside slow cooker.
Mix together brown sugar, vinegar, lemon-lime soda, garlic, soy sauce and pepper together. Pour over chicken. Cook on LOW for 6 to 8 hours or on HIGH for 4 hours.
Take chicken pieces out of slow cooker (they may fall apart) and pour remaining sauce into saucepan. Place saucepan over high heat.
Mix together cornstarch and water, pour into saucepan, and mix well. Let sauce come to a boil and boil for 2 to 3 minutes or until it starts to thicken and turns into a glaze. Remove from heat and let sit for a minute or two (it will continue to thicken as it cools down).