The 2019 Rear window frames were prone to cracking,
2020 window frame seem to be OK
If the roof liner is stained above the window, the 3rd brake light seal is likely the cause
but,
there is a slight chance, that the roof line rubber seals could contribute
If the water stain is along the bottom of the Sliding window, the window Track can get filled with natures trash.
Like pollen, pieces of leaves, dust, a touch of sand, etc.
There are 1 or 2 holes in the window track.
The drain exit holes are built into the window seal rubber.
If you use a plastic scraper, and run it along the bottom of the rubber seal outside, you may open up the holes exit.
In one video a guy was using something to squirt water into the window track, it was not enough to overflow the track.
Then outside he used a scraper to separate the rubber from the paint.
That allowed the water to drain & run down the exterior of the cab
My window was leaking very bad. And I read and read online but no videos that I could find. Hope this helps for guidance.
www.youtube.com