While working on the diff plug I had to jack up the rear RH side to crawl under and see if it was seated.
Water poured out the front of the RH sill via that fold in the bottom edge .... I thought I had sealed every hole in the car except the doors ! Have not been driving in the rain but it has rained heavily on it .
No water in front footwell or behind the seat but yes water under the RH side back seat .
Wiped it up but it came back instantly ..removed a small plug in the centre and it was coming from that ..double skin it seems about 1/4 inch away . Strange I could see the ground through a "puncture hole" in the lower skin..Has someone been here before. Put some welding wire tho the hole and will see where it comes out .

Anyone know of the construction in this area ? double skins etc or had this problem?

 

If its a coupe, when I was fixing all the leaks on mine, this turned out to be the rear quarter window seal. Took trim off and put sealant in there.
Could also be a mixture of the top waist seal and front windows not sealing, but usually rear carpet would be wet as well.

Something else to note, if its on the filler cap side, it could also be leaking from there.
The factory sealant on mine from the inside rear quarter was letting lots of water in and needed to use tiger seal to plug back up.

Can't think of any other places of the top of my head, good luck!

 

Have sealed rear quarter light on RHS and it seems to have stopped