This Mini was leaking rain water into the boot, and from there the water can get elsewhere.
We found the cause of the problem to be that water splashing up from the road up under the back bumper is running down the rear valance, and getting past the seals on the rear air vents and finding its way into the box-section which is inside the car.
Very many cars have these hidden rear air vents which are designed as a one-way outlet so that if the heater is blowing in air from the front of the cabin, or you drive along with your windows open, the air has somewhere to escape out of the back. So there are quite large air vents which are usually hidden behind the back bumper. It is not unusual to find that these are the cause of leaks on many makes and models of cars because they are not only hit with water splashing up from the road, but all the dirt and pollutants to which can either gunk up the gate or perish the silicone seal.
Solution: Remove the vent, clean up the area. Replace the vent.
Question: "So you just stick the vent in and seal around it to stop the leak?"
Answer: You could do that, and many of the garages do this... but it's not what we do. We prefer to restore cars to factory standards, so we replace the vent with a new part. Usually these parts are not expensive, on some models the fault is so common we actually keep them in stock. Because the manufacturers are aware of the leaking vents, some have even improved the design to prevent future leaks.
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