Defogger Heater problems of the 1998 Ford Windstar

The problem seems to be related to a blend door which blends heated air with a/c. The door apparently does not position properly and so a cold mixture is delivered o the front compartment. The rear ventilation controls work perfectly (this is the extra capacity heat & a/c option). Repairs have been attempted in severn visits to two different dealerships over the past eight months but the problem still persists. Although the blend has been replaced the problem has not been fixed. The dealer indicates that this is a problem that Ford needs to fix. Ford has not accepted responsibility for the problem to date and I am without heat in the front compartment.