If your brisket comes with a seasoning packet, throw it away. You won't need it. Put brisket in crockpot and cover with water. Cook on low for 10 hours.
Turn on oven to 400 degrees. After cooking in crockpot, remove and pat dry (careful! it's hot). Mix together the vinegar, molasses, horseradish sauce and mustard. Brush to coat brisket and bake for 20 minutes. Slice and serve.