Use a 6-quart slow cooker. Place the brisket into the insert. Add the onion and garlic. In a medium bowl, mix together the cranberry sauce, juice, soy sauce, and dried cranberries. Pour this mixture evenly over the top of the brisket.

Cover and cook on low for 8-10 hours or on high for about 6 hours. The meat in finished when it has begun to lose shape and can pull apart easily with a fork. Serve over mashed potatoes with a spoonful of the sauce.