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1st thing to check is the state of the rear light seals as there have been issues with these not sealing - if you remove the trim in the boot so you can see the back of the light and then play a hose over each one and inspect the light back to signs of water ingress
If the car has AC then this could be the AC drain pipe thats come adrift or is clogged under the car and backing up the system.
To get to the pipe you access it from under the car
Also worth checking that the door and hatch seals don't have any rips or tears and that they're touching all round the door - best way to confirm this is to rub some vaseline into the seal and close the door then when you re-open it check that the grease line is all the way round and check where theres any gaps - as this indicates the seal isn't touching
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