Mildew Smell when I first get in my '19 1500
 

I have a 2019 Silverado Z71 Crew Cab with 37K miles. I started smelling a mildew smell roughly 6 months ago (maybe 5K miles back) and I thought it was from some boots I left in my truck and it was a very light smell. However, the smell has gotten progressively worse and I can't find any moisture, wet spots or actual mildew that is visible in the truck. During the last cold snap here in KY I did find a spot in the back corner of the rear window where there was some water actually frozen on the inside corner of the window (I took pics to show Chevy). I tried to put the rear seats down to see if I could see any signs of mildew behind the seats but I guess my seats don't fold down.

The smell only seems to last for about 10 seconds when you first open the doors and get in and it seem to be the strongest when you open the back door on the driver side but when I smell the carpets and smells around the seats in that area I smell nothing after the initial shock of opening the doors and smelling it. I have an appointment with the dealership to have my transmission looked at for the 8th time (I'm at 37K miles) and I have asked them to try to determine where the mildew smell is coming from. Any suggestions or thoughts? This started while I was well within the 3yr / 36K but I have no idea if they will cover it given that I realized it was a bigger problem and reported it around 37.4K miles.

Might be a wet cabin air filter.

If you have the rear slider window, there's a chance it could be leaking from there. It's a known issue on 2019 and newer trucks. There's almost 300 pages of forum posts on the gm-truck forum talking about it with varying fixes if in fact that is your issue.