Martian

probably power for emergency lights

71HntrGrCntnl

Common problem on F-150 for many years

grss429

what year truck? also can you take a closer pic of your sliding glass picture? I'm having similar issue

as you see in my last pic it looks like part of the stripping has shrunk causing a hole there

https://www.f150forum.com/f4/2006-f1...w-leak-323199/

CollinM

Guarantee its the third brake light. Either order new or clean mating surfaces and put a new rubber seal in instead of the foam

2nd150

Try removing the light and look for signs of water in the cavity..If there is any, most likely the light is the problem. It's easier to reseal the light than the back window. When you put the light back on - after it's screwed in- put a small bead of clear silicone on the top 3 sides and in the screw pockets.

Edit: OP, I would first suspect the extra grommet/plug in the roof.