I've been having the carpet get wet back there anytime the car stays outside and it rains. I've poured water down the sunroof drains and they seem to be draining properly. I had the TSB for the silicon done around the grommets back in 2007, perhaps it needs to be redone?

What's the easiest method of tackling this as there doesn't appear to be any drains in the pillar there! Remove the front seats, center console and rear seat cushion and carpet to figure it out?

 

Have you checked the B pillar rear door seal in case it's coming from the door corner and not the sunroof? Since it happens while the car is sitting.

 

I will check the whole door seal and blow it out with some compressed air!