You can see there is a plug at the bottom of the seat base metalwork, this looks like it has been sealed from the factory but when I touched it it lifted straight up, this was the same on the other side also as it looks like the sealant has gone off. Under this plug is the fuel tank and I have found under heavy rain water settles on the top of the tank as you drive and can flick up the plug and this is how the water is getting in.
To fix this I brought some grommets and where a tight fit into the hole (its 35mm) cleaned the metal work and applied some silicone sealant to outside lip also.
Once the grommet is fitted I also applied some more sealant to make sure there was a good seal.
A couple of days later and a lot of heavy rain and going though puddles I can confirm the rear is as dry as bone. This is for a D8 estate but not sure if the other models have the same floor pan but worth checking, the seats were always dry its just the soundproofing that was soaked.
