'08 SCREW, same problem, coming from up front somewhere. 3rd brake light is siliconed, plus I caught the puddle spreading front-to-back after drying it all out. Still didn't see where it was getting in, though, just not from the back.
I bought the AC drain elbow, which was $18 and black rubber, not aluminum - wonder if I got ripped off. Tip: also get two standard black plastic "push pins". It's well worth a buck or two to just cut the old ones, one in the top of the liner just forward of center, one in the extreme front outside corner.
I pulled the right front wheel and started removing the liner. The area behind the lower rearward section of liner, i.e. directly behind the mud flap, was completely packed solid with leaves and dirt and muck, sopping wet. I suspect rain water coming down from above was backing up behind this muck and finding its way into the cabin floor.
There's also a sort of boxy diagonal brace above that area, behind the liner and below the hood hinge, with drain holes out the bottom; this too was full of dirt and leaves and a sopping wet wad of firewall insulation (mouse nest?). I used a bit of coat hanger to pick out as much crud as I could through the drain holes, then ran some water in from above with a hose, which washed more crud into the holes; lather, rinse, repeat until clean.
I installed the AC drain anyway - in for a penny, in for a pound - but suspect my leak was almost certainly caused by those two areas behind the liner not draining freely due to years of accumulated leaves and muck.