You should still have Pos 4 working so check the relevant fuse first, what can happen as said is if the pollen filter is not sealed correctly water will leak past/through it onto the heater motor itself thus seizing it, this is what causes the fuse to blow, if fuse is good I would be checking the switch next.

Awwaffic, it sounds like you have been unlucky as it sounds like the resistor pack has gone again, I would take it back for a replacement, did you get it new?

Martin