so i just discovered that my third brake light is leaking. And i know i've seen it mentioned on the forum somewhere. But my question is, can i buy just a new gasket for the light or do i have to buy a whole new light?

 

Go to Lowes or Home Depot and buy some window weather stripping with one side adhesive. Take the light (2 Phillips screws) and gasket off. Clean the light and put the weather stripping on where the gasket should have been. Join the weather stripping (joint) at the bottom side of the light. Throw the gasket away. I don't recall the exact size of the weather stripping but it wont be too difficult to figure out. Cost just a few $$$'s.
Mines been like done for a year and a half - no issues so far.

 

There is a service bulletin on it, I believe it states to replace the whole light. I just removed the light and used RTV under the light to seal it up, if done right none is visible from the outside.

 

RTV used here too with no more leak. Granted it will be a PITA to remove if I need to replace the light, but I'm not too concerned with that on an 04 F150 and a third brake light law that is rarely (if ever) enforced where I live.