Something is causing a big draw to blow the fuse. It souds like a faulty switch or motor. The switch is an easy check assuming the power goes via the switch to the fan. I would disconnect the fan side lead and then use a bulb or multi meter to see if it works in both positions.
The motor I'm guess if like my 97 +4 isn't easy to reach. Others may have more experience & some ideas
A word of caution though the switch may have failed due to the motor pulling a big current os replacing it alone may not fix it.
