Hi Guys,
I've had the same problem with water in the tyre well. No great surprise I traced it back to leaking pressure vents on both sides.
Basically the 'reservoir' below the vent slowly fills up with water and then overflows into the tyre well.
To get to them you have to remove some bits of trim. The key bit being the rear trim to the luggage compartment. This video shows how to remove it:
The removal of the rest of the trim is fairly straightforward.
I put a good seal of silicon around the edge of each vent.
As a 'belt and braces' exercise I also drilled a small drain hole in the plastic bung just below each vent. Should the vents ever leak again, then the 'reservoirs' will self drain to prevent any overflow into the tyre well.
Fingers crossed that I've solved the problem!
Peter