When my motor failed it was working one day and failed the next. I think when the resistor goes the fan will still run at full speed. It is only the lower speeds (when the resistor is in circuit) that is affected. If the Climatronic is calling for low speed you will get nothing but it runs on high speed when that is called for. Set Climatronic to Demist and see if it runs then.

I 've just tried the demist and it immmediately increased the speed to max. So the resistor should be ok ? Maybe the motor will fail slowly ?

No. If the motor runs on demister then the resistor pack is faulty.