water leaking onto back seat of 06 sierra ext. cab rear window or brake light?

[mc1984ss]

I have had my 2006 Sierra extended cab about 3 or 4 weeks and it got its 1st heavy rainfall today. I turned around to look at my son in the back seat and noticed the middle of the back seat was wet. It was coming from where the top of the back window and the headliner meet. I was hoping it is the 3rd brake light leaking and not the window. Either way it did not make me very happy and has me very concerned. Is there a sure fire way to tell what is leaking? Are either the window or the brake light common problems on this year range? I want to fix it right and dont want to put a "band aid" on it Thanks in advance I really appreciate any help. If it is a simple 3rd brake light fix I would rather not take it back to the dealer they have not made me very happy. Thanks again!

[SpiderGearsMan]

sliding window ?

[SpiderGearsMan]

we had a lot of problems with leaky sliding windows

[Brian Lohnes]

If that window has ever been replaced, it is probably the window that is leaking. My extended cab Ford had the rear window replaced when a guy that works for me put a 2x4 through it. The glass company had to do it three times to get it so it would not leak. This is a single pane, non slider.

[mc1984ss]

it is not a sliding rear window

[Stich496]

so reseal the brake/cargo light

[SpiderGearsMan]

we water test with a garden hose

[mc1984ss]

i mistyped my reply above I accidentally typed it is not a leaking rear window I meant to say it is not a sliding rear window. I got a chance to look at it and it appears to be leaking from the thirb brake light. i do not want to gob a bunch of silicone in there, I would like to fix it right. There is a foam gasket from the factory but that obviously is not doing the job. Is there a new gasket available for the brake liught? I am having a hard time finding one? Thanks again