I know I had the same kind of issue with my wife's equinox and as it turned out it was the sunroof leaking and running down the "A" and "B" pillars and laying under the seats. But I don't know if you have a sunroof or not

 

Nope, no smoking. Smell not from ducts, it was/is from the carpet padding.

No sunroof.

And we have had some bad rain deluges "wrath of God" rain in last 2 months since I found the problem. Yet no more wet padding.