The back window on my 04 Supercrew lariat has a waterline the whole width of the top of the back window on the headliner. It looks like maybe the seal is going bad or something. The headliner has a waterline where it has gotten wet for some reason.

 

Sounds like it could be your 3rd brake light leaking. Common problem with f150's. Will present exactly where youre describing. Pop it off and hit it with some silicone.

 

but would what be causing the entire width of the window to be getting wet, not just the middle???

 

A lot of water getting over a long time.
Is there condensation on the inside of the light? I have an 07 and that leaked the same way,very common issue.Take the light off and put a thin bead of silicone right on the gasket and reinstall--I put a blob in the recessed holes for the screws after I snugged the screws down. So far no condensation and no leaks.

 

There is just a little bit of condensation in the clear ones. I replaced the factory bulbs with led ones and dried out the housings with a heat gun. Put some silicone around the part of the socket that screws into the light. That has helped keep moisture out of where the bulbs are. Is there any special brand of silicone that i should use for this or doesnt it matter?

 

Any clear silicone is fine, just do the whole way around the gasket.

 

would it hurt to put a bead of silicone between the cab and the window itself along the top? or is that a bad idea?

 

Probably not necessary..just end up makin a mess. Try the light first, that will likely take care of it.

 

x2----its the light mine did it about a month ago, some RTV and all better

 

The 3rd brake light is a common source for leaks across the truck spectrum in general. The weatherstrip/molding tends to errode from exposure to the elements and sun (UV rays), thus lending to a loss of seal. I had the same issue on my Chevy [sold] and I used silicone to eliminate the leak.

Silicone comes in black and clear (and other colors for that matter) and since it looks like you have a red or maroon truck, I'd go with clear. First, look closely at your brake light and see if there's any condensation, as you'll want to make sure that the light is dry before you start sealing it with silicone (even though small air pockets will remain on the inside where the water gets from the light to the cab). If you have water droplets inside the lens assembly, then you may need to remove the light and use a hair dryer to clear it out.

Word of caution: start with a small bead of silicone and you can spread along the lens edge with the cap from ballpoint-pen [the tab part of the cap]. I applied a little too much on my truck and it made a bit of a mess: lesson learned.