Strange Issue - 1999 Chevy Suburban C1500 2WD

[shultzee13]

Bought this vehicle used 2 months ago.
Had the carpets cleaned several times since.
My problem is behind the front passenger seat the carpet by the lower air vent stays wet (damp).
The ghost is I have never ran the rear ac. I put a piece of paper down there just to see if anything is blowing out and the piece of paper never gets wet.
So I believe whatever is keeping the carpet damp must be from underneath.
If it's leaking from the cooling system under the dash NONE of the front carpets are ever wet.
The only place in the whole car that is getting damps is behind the passenger seat.
Is there anything under the carpet there at that spot that could be leaking?
Thanks guys.

[lazerstrouts]

the rear a/c unit is behind the right rear quater panel trim behind the rear wheel. could it be that they arent getting all the water out after being cleaned? the paper is under the black plastic vent correct? do you have a sunroof?