citroen XM 1998. The heater blower on fast all the?

The heater blower runs on fast all the time once the engine has started. 2 years ago a new control unit (diode pack?) was replaced. this lasted 18 months and was replaced again (at vast expense- nearly £200). Now it has gone yet again after only 7 months. This time the part is under warranty so I'll get it free apart from fitting cost. How can I prevent this happening again? Is something else at fault or am I just unlucky with faulty parts?