Failing shutting someone in the boot with a torch to look for leaks while water is poured on the car then strip out any/all trim panelling[including boot carpet] and chuck some water at it-hosepipe is best but watering can will suffice-if the leak is not immediately obvious do this for a few minutes and check for leaks.Make sure you check around the rear lights in case the seal around this/these have gone.Check also around the edges of the boot floor[in case the water is coming upwards]for any holes/break in the seam sealer.
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