Thankfully this is an easy fix. The resistor pack gets crud built up on it from being in engine bay, exposed to the elements.
First step is to remove the rubber strip that hold front of cowl over heater blower fans
Once that’s off cowl lifts up at from and you can access resistor pack and disconnect black plug for wiring
Then remove the 2 screws, one at each end.
Once out remove rubber gasket and put it to soak for about an hour in white vinegar. I used a toothbrush to give it a light scrub too
After it’s soaked for an hour let it dry(I used kitchen paper towels to speed up drying). Once dry, refit and you should have all fan speeds working 👍