Wet cargo floor 2007 Suburban

I have a 07 Suburban with a soaking wet carpet in the rear cargo area. It seems that every time its wet outside water is coming in from somewhere and has soaked the carpet and insulation. It's been to the dealer 3 times already and they have changed the weather stripping on the back gate, but the water still comes in. It wasn't too bad in the summer, I was able to clean it up when I first noticed it but now it's a block of ice.
Any Ideas?

Does your Suburban have a sunroof?
If so; it could be that the rear sunroof drain tubes are plugged from improper installation at the factory.

Yes it does have a sunroof, never thought of the drain tubes. Next time I have it in a warm place I'll funnel some water down the drain to see where it comes out or if it leaks.
Thanks

Look for it to leak just behind where the C-pillars meet the floor, the water can travel rearward from there. 

i had this problem on my 07 Yukon XL.  turns out that there is a thread sealant on the bolts that hold the roof rack down.  I switched my stock rack to an Escalade roof rack and forgot to put sealent on one of the bolts.  so water would come in thru that bolt hole, hit the top of the headliner and run back and drip out of the uper moulding.  this happened on the passenger side all the way back near the tailgate.  it was an easy fix once i figured it out, but like you, i thought the rear door gasket or window was leaking