fixed mine two weeks ago
I had the same problem, water accumulating in the foot well of the LH (driver'* side) rear seat. I pulled the door panels to check the butyl seal on the water shield (big piece of plastic sheet) and both were bad. I fixed that (go to a store that sells body shop supplies and buy the strip of butyl they sell for installing windshields in older cars). Left the car out in the rain, and the problem was still there.
Pour a small amount of water into the front and rear drain openings in the sunroof. For the front ones, move the sunroof back a little to expose them. For the rear ones, pop the back of the sunroof open to the vent position. Pour the water in one drain hole at a time and observe where it drains out to the road. If it comes out anywhere other than the wheel wells, you have a disconnected sunroof drain tube.
My front left drain tube was disconnected behind the IP.
Remove the front tire on the side where the leak is, and remove the rear fender liner. You will see a small grommet with a nipple on the rear of the wheel well behind where the liner was. This is the outlet for the front drain tube. Work the grommet out with a screw driver and give it a gentle pull. On mine, it just came out which meant it was disconnected.
As a side note, in order to get my carpets completely dried I pulled the front seats, the console and the carpet. When you do that, you can see there is a channel in the floor board that allows water entering near the IP to flow right back to the rear seat foot well.
Do not try to reconnect the drain tube, you can't get at it. It is easier to replace it. Let me know if you think you have to do that, and I will describe how. It is not that difficult and only cost a few dollars.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)