I would unplug the motor, and then operate the fan switch, and see what the result is, if the fuse remains ok, you know the motor is at fault, seized most likely, or a wire shorting, but could even be something jammed in there, esp if someone has used it without a pollen filter if it should have one.
I remember my renault 11 used to blow the blower fuse if I went round right hand bends to quick, the rag I was using to check the oil etc was being stored behind the firewall bit, and used to slide across the fan input and jam the fan!!
tabetha