We all know seals wear down over time and it's usually not a big deal to just go down to your local shop and pick up a $5 piece and be on your way to replace it yourself... Well the same is not true for 2004 & newer F150 third tail-light gasket. When this thing starts to warp water actually pools along the bottom of the door frame (with a lot of wires running to the fuse panel) potentially shorting out all kinds of fun stuff. If you go to ford and ask for the replacement gasket you'll be pretty disappointed when you find the $5 part is actually not for sale on its own, but you have to drop over $100 to replace the entire light fixture. It doesn't take a rocket scientist to figure out there's no reason to replace the entire light when all you need is to seal it up again.
Now most caulking will do the job, but for this particular application I used "Dum Dum Caulk Strips" because the stuff doesn't crack or shirnk so you don't have to worry about replacing it again until you actually do need to replace the light ficture. One other thing to note is be sure to use a small O ring on the screw as well.