Water ingress to rear seat area driver side

[Big martin]

My Celica rear seat base is soaking wet. I have sealed the round black grommet near to the tailgate hinge but it still occurs. 
No sign of boot floor getting wet that I can see as mat is dry. Anyone had similar and successfully sorted the problem. Thanks for any pointers. 

[Big martin]

I’ve found the problem. Removed interior plastic trims along with the rear seats and got the hosepipe out spraying the roof. Drips started to appear on the rear C- pillar, drop down near to the rear seatbelt and run along onto the floor pan next to the wiring harness. I have removed the exterior black plastic gutter trim carefully and sealed the seam that it covers with seam sealer. Then refitted the trim. Tested with hosepipe again once the sealer has dried. This has stopped the water ingress. Hope that this helps others if the same should happen to their cars. 🙂👍