Hello
I think I might have sorted mine out.
I have been playing around with a jug of water and my boot lid to try and find out where the water was getting in and it looks like it's via the rubber seal on the rear wiper.
This seems to be a bit loose and hardened which left a small gap for rain water to run into the boot lid and on heavy rain into the boot and wet the carpet.
I removed the wiper and the rubber and resealed it with a small tube or instant gasket.
I then replaced the rubber while it was still wet and it seems to have done the job.
Let me know if it works for you.
Thanks
Dave