Z3 Roadster (E36/7) Heater Control Issues

Hey!
I really do hope that Bimmerfest will be able to help me with this problem!!!
I own a 1999 Z3 Bella Machina.... and have been struggling with its heater control for quite some time now, even had it with the dealers without success.
When I turn the heater control on, no matter even how slightly, it automatically goes onto full heat.... that is still okay, but then it simply doesn't go off again, and does so in its own time... which can be anything from a week or two, or a month or three.
It is useless to turn the air conditioner on, as the flow of heat is so strong, it simply overrides it.
Once she has decided by herself to stop blowing hot air, the air conditioner works absolutely perfectly.
Firstly, where in the engine can I go to turn the flow of heat off manually, and secondly what can I do to repair it if not even highly skilled Mechanic cannot find the cause, nor a solution to the problem.
Your assistance will be greatly appreciated, as you can very well imagine the discomfort with outside temperatures in the mid 30's and full heat inside the car.
I am looking forward to members' help or advise on this!!
Kind regards
Derek