In a slow cooker, cover corned beef with water. Cover and cook on low 10-12 hours or until tender. Drain corned beef; place on a broiler pan or ovenproof platter. Preheat over to 400F. In a small bowl, combine mustard, horseradish, vinegar and molasses. Brush on all sides of meat. Bake, brushing with sauce several times, about 20 minutes or until meat begins to brown. Cut into thin slices.