In medium saucepan, bring water and 3 tablespoons margarine to a rolling boil. Remove from heat. Stir in milk and potato flakes with fork until well blended. Set aside.
Melt 2 teaspoons margarine in medium skillet over medium heat. Add onion; cook and stir until tender. Stir in chicken, taco seasoning mix and 1/2 cup salsa. Bring to a boil. Cook 2 minutes, stirring occasionally. Stir in cheese and potatoes.
Spoon about 1/2 cup potato mixture down center of each warm tortilla; roll up. Top with lettuce and additional salsa.