I had a very similar problem , found out that it's water collecting in the bottom off the fan housing where the wires connect to the fan . If you look under the dash in the passenger footwell behing the glove box you should be able to see the cable that goes into the plastic moulded housing off the fan housing , The wires pass though a rubber type grommet All I did was prize out the grommet and clear out the bits of leaves and dried out the connections , havnt had the problem since . Worth a try and costs nothing .
Here's a pic
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