Blower Transistor

I recently had my Blower Transistor (climate control version) go, I had it repaired but it has gone again, fan on full speed and wont shut off, anyone any ideas as to why this may have gone again.?
Been in touch with company in Italy who made this part who sent me drawings of this part i need and a list of dealers i can get it from so may be able to get new one but not holding out much hope.