Nothing, that is how the defrost works. When you turn on the defrost the A/C turns on automaticaly. If you mean that the air blows out of your "vent" air ducts when you turn on the A/C then you may have something stuck in your air duct plentum or a air duct plentum door is not working. This type of repair involves removing the entire dash board and air ducts to just find the problem.
Dec 4, 2019 at 8:28 PM