I posted this on the C30 forum, as well:

My '09 C30 has developed a leak, somewhere on the passenger side. Soggy carpet on the floorboard, behind the passenger seat, after a rain last night. I dried it out pretty well with a 24 hour fan, last week, after I first noticed it.
Both the door seals seem fine, as does the moonroof. The tail gate flip-up window will need a new seal, but it's still in one piece. Can't see how water could get around it, then run down, and several feet, to the floorboard.

Thanks

 

Probably not much help as it's a different car but my 740 has a hose that leads from the cowling by the wipers into the plastic housing for the blower fan. I replaced the blower motor a couple as it was rusty. Last time I left part of the housing off so it can drop on the floor rather than collect in the motor. For the life of me I can't imagine why they wouldn't just drain it to the road but there is definitely a hose from the cowling to this plastic housing for the blower

Maybe you can see if leaves have blocked other drains from inside the cowling area? Blow it out with air maybe?

 

I don't know if this is particular to just the C30, but the sunroof glass doesn't retract fully. This prevents me from getting to the drain holes in each rear corner. I suspect one corner is clogged, resulting in the water buildup overflowing down the side pillar and onto the rear floorboard.
Any ideas or suggestions?

 

Try sticking an air hose into the drain hose and see if you can blow the stuff through. You can invert a squirt bottle with water behind the air jet. blast the water in through the hose.

Or you can try to stick a piece of hard wire through and then you can pull a bit strong string with a little strip of cloth

Cherry pits and things get in there

 

I cannot get to, or even see, the drain holes. I looked underneath the car, in the pillar area, behind the passenger door. Could not see an outlet hose.