Anyone else see this issue? I haven't determinsed whether it's only when I drive it in the rain or even when it's parked during a storm, but it gets really damp. My Cousin has a 99 Sierra EC and said he has a similar thing going on but hasn't looked into it either.

Start with this:

Open door + rear if you have the extended cab.

pop up the plastic trim piece between door and carpet, then remove the one or two screws that hold the plastic cover surrounding the Parking brake/hood release.

Now you can pull up the carpet on the drivers side.

In my (2002) there are a few grommets in the front wheel well area that I noticed when running a 2 gauge cable for my amp.

Maybe one of thise is woor/ripped/missing?

Good luck..

Maybe put a sprinkler (or have a friend) spray water to try to find it?

The plastic wheel well cover pops out after removing the plastic expanding grommets. There's a slot for you to get a screwdriver into to pop the head of the grommet out; and then the body will come out. Take that off and look in wheel well for things that look out of the ordinary.

i had the same prob on my extreme blazer, but i sold it before i could fix it

I have the same problem with my '99. It's for sale. ;)

Mine is almost always damp.......but I think mine is or actually was because of the floor mats......now I have the catch- all extremes......so I will probably have to check to see about it ......

possible leak at windshield whiich drips onto the carpet. I had a few vehicles that did this

could be the door seal, or could be the door boot where the wires go through, could be windshield, or it could be a grommet in the firewall.
is there anything added to the vehicle that you drilled holes or anything??

My passenger side carpet did this. The seal around the 3rd brake light was not sealing corretly and it was running all down the side pillar where the speaker mounts. My floor was soaked. I had to remove the light and use some clear silicone to seal it up. I then removed the entire interior of the truck and let it dry for a few days.

I have a 2000 z71, reg cab, my drivers side carpet was wet when its a heavy rain for a long time, i have a catch all mat, by luck i noticed the third brake light leaking one day.
Re-sealed it and that solved the problem.
good luck