Passenger side leak.
I had same problem, it will be getting in through the nearside coverplate, one of the fixings becomes loose and "wicks " the rainwater straight down the heater blower, it you check the blower in the passenger footwell, you will see water dripping from it.
Solution.
1 Raise bonnet
2 Remove wiper arms (tool needed)
3 Remove plastic pins that hold the black valance in place.
4 Remove valance and you will see a black plate on the passenger side, held down by 4 screws.
5 Remove screws and lift plate, seal with mastic, apply mastic to all screws especially lower right, as this is the source of the leak.
6 Reassemble etc Job takes about 45 minutes, but you may have to fabricate a wiper extractor tool to release the wiper arm. Look on ebay first to see how they are made, then bend a bit of metal to suit.
Hope this helps. I did take some pictures at the time, but not sure how to upload..
Best regards, Rustic