1999 Z3 Bella Machina.... When I turn the heater control on, no matter even how slightly, it automatically goes onto full heat.... it doesn't go off again, and does so in its own time, Once stopped blowing hot air, the air conditioner works absolutely perfectly. Where in the engine can I go to turn the flow of heat off manually, and what can I do to repair it if not even highly skilled Mechanics cannot find the cause, nor a solution