I travel for work and put in many hours, so I didn’t have time to look at this myself.

I took my 2016 3500 to a dealer to have it looked at.

The headliner was getting damp near the back rear corners every time it rained.

The service department charged $140 to diagnose.

They said the factory rear window seal was leaking and the whole window needed to be replaced and quoted me a $2000 estimate to do the repairs.

They said it was impossible for them to re-seal the window because they couldn’t guarantee it wouldn’t leak again.

The headliner was getting wet ABOVE THE REAR WINDOW.

It didn’t make sense that a large amount of water would ‘wick’ upward into the headliner.

I told them, ‘no thanks.

I took the truck back home and noticed the headliner wasn’t installed correctly, and there were greasy fingerprints all over.

I took the truck back to complain, and they cleaned the headliner and detailed the truck - a nice gesture on their part.

I took the truck back home to have a closer look at myself.

After a quick google search, I found the 3rd brake light seal was a common culprit.

I watched a couple of Youtube videos and saw that others were using a $9 piece of weatherstrip to replace the factory brake light gasket.

I pulled the light out and could see evidence of water staining in the center bulb and below where the bulb fits.

I could also see a metal channel running under the brake light that extends toward each side, explaining why the headliner would only get wet near the back corners.

While I was scraping the old gasket material off, I noticed that water would come out if you squeezed the rubber.

After I replaced the gasket - no more wet headliner!

The cost was minimal, and the whole repair was like 15 mins.

Posting this in case someone else runs into the same problem.