After further investigation into this Suzuki, James has pinpointed the exact welded seam which is causing the problem and allowing water to leak in.
Leaks like this are fairly unusual, but not unknown. It could be that at some time in its past this car has had a crash, or it could just be time that has caused a seam to crack, as the shell of the car is under constant stresses as you drive around corners, causing the chassis to twist and flex. Either way, the solution will to be to take the wing off, (you can see where it is bolted on) and seal any seams we find underneath. Welded seams are not waterproofed by the welding in most cases, as they are often spot welded. They are sealed at the factory with a gum which fills the cracks, this can dry out and crack over time, which may well be all that's happened here.
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