Third brake light leak

I apologize for what I’m sure is a common post, but I’ve done a ton of research and am not any closer to solving my problem. It appears there’s the infamous leak coming from that light back there.
I’ve replaced the rectangular gasket with a dealer part, put sealant on every seam I can find, and replaced the screw washers with new o-rings. It’s still leaking (and Texas is rainy AF at the moment). Is it possible the rear glass is the culprit? Am I missing something?
Ideas are welcome.

 

Is it going through the headliner? Possibly try using some RTV?

 

It is. Dripping into the cab.

RTV was the second thing I did.

 

Your year doesn’t have shark fin antenna right? Best bet is to pull off the headliner…

 

It does not.

Drop the headliner, then what?