how can i fix my y reg ford mondeo 2.0 hdi d?

when i turn my fan switch on th first two setings it doesnt seem to blow but when i put aircon on the switch light comes on ,but when i put fan on to the thrd setting it starts to blow and the aircon goes off. it never used to, as any one ecsperieced this with their aircon system. is it because the fan doesnt blow on the first two settings ??? can anyone help me please. thanks.