Do you have a sunroof?
Have you powerwashed the rear lights recently? If so, the dirt that would have been lodged and therefore sealing the light could have come loose and so it's leaking in.
Another one I believe is each time you open the boot after it has been raining have you noticed the rain spills off the boot and then directly into it just above the lights? Now when I open the boot I open it halfway for 10 seconds and let the water slide off onto the ground before opening fully. Anyway, I think there's a drain valve under the spare wheel that can clog, remove wheel and check!
Finally, it's probably just natural water condensation. Every time you get in with wet boots or whatever it may be the water stays in the car and condensation naturally builds up. Once it was so bad in mine I used an industrial dehumidifier placed in the boot with the back seats flat and left it running for a few hours before taking over a litre of water out. To avoid the condensation building up on the windows in the first place apply washing up liquid (one tsp) per window with a dry microfiber cloth. Leave it for a few mins and then buff off with another clean microfiber. Don't dilute the washing up liquid. It leaves a hydrophobic coating which prevents condensation on the windows. I usually reapply every 2 months!
