cold and fed up

Hi all can anyone please help me? I have a 2002 tdci fiesta with heated blower problem, ie it only blows on no:4 setting and is noisy. Today i changed the resistor and got the fan working but it was noisy still and only lasted about half an hour before it went back to only blowing on no:4 again. What can causing the resistor to blow so quickly any ideas? I didn't smell burning if that helps, it just stopped working again. I am getting desperate as i have to drive it at 6am every morning and don't how i am going to cope with ice on the screen and misting up.