Welcome to the form mate. The water is getting in via the pollen filter in the scuttle panel. There is a large rubber drain grommet that gets blocked with dirt and leaves. What tends to happen is it blows the resistor that control's fan speed 123 but not 4.
It could be its starting to seize?
Do your dials also drop when the dash lights stop working? If so try spraying your aBS plug with some contact cleaner or WD40.