Have you checked you blower motor?

The cover over the pollen filter is a poor design and relies on the double sided foam seal to prevent water running off the windscreen into the air intake. When it does the usual symptom is a wet footwell. However it can also get into the blower motor and cause corrosion making it stiff.

Still takes a lot of current but little airflow, so the thermal fuse on the resistor pack blows. That can be replaced with a near equivalent from Maplin for a few pence.

Martin