i've got a similar problem with mine, 2001 focus estate.
at some point, windscreen must have been replaced and they hadn't stuck the cowelling back properly on the passenger side. When it rains, water pours under the seal into the air duct - straight into the blower. awesome design. i've used clear sealant to stick mine back down. passenger side carpets damp at all? thats water getting in that side.
Anyways, now i have a noisey blower, bearing probably rusted/gone so i need to sort mine - but i've done loads of googling recently on repairing the blower. i'm not risking a 2nd hand job off ebay that might already be chuffed - so i'm just going WD40 it!
Anyway, 1st check the fuse, make sure it's ok. Next, get a multimeter and check the switch. it's probably not these but always check the obvious first. if they're ok, DB's probably right - it's the resistor pack. As the fan wears it burns them out. u can get a new, better resistor pack off ebay OR if you're handy with a soldering iron i found a guide via googlewhere someone got a replacement resistor for 79p from maplins (i find it if u want it)
Getting to it all is a mare, basically, take out the glove box and start from there.
i might do mine this weekend as no blower in this weather is a mare!