I couldn't believe how expensive is the factory third brake light, so I decide to fix mine... I'm not in the give away money... here is a way were you can fix your light if you have the patience and the MONEY... UNDER $5.00 REPAIR
I'm not sure how many pictures I can post at one time but I will try to explain how to do it...
Look at the outside part of the light to see if the lens is glue to the housing, cause some lights leak thru the lens beside the gasket leak...
When you look at the back of the light you will se the gasket is even with the housing stoppers, meaning that the gasket shrunk and there is no more room for the gasket to keep a tight seal.
Peel off the gasket from the housing slowly with out ripping the gasket...it is glue with a thin double side clear paper. you will notice that the gasket was indented into the housing in the middle and there is not much flat surface for the gasket to sit on it, you will seal from the inside middle groove to the back flat part of the housing raising the groove to 3/16 higher than the housing gasket wall.
look at pictures

 

second set of pictures. when you place the sealer on the housing try to be as steady as you can with the even line of seal front the front of the groove to the inside of the housing
let dry for around 15 minutes and drop the gasket in place with out putting pressure on the gasket just rub your finger slightly over the gasket and let sit for around 15 more minutes
also if you want to seal the lens to housing do it now (optional)
DON'T PUT ANY SEALER ON TOP OF GASKET OR ON THE EDGE OF LENS
Remember that sooner or later you will need to replace the bulbs and you don't want the light stuck glue to the cab ( unless you don't care of extra hard work removing the light)
when you are ready to set the light on the truck ONLY shows the screws to lens with out tighten them at this point you only want to put flush the light with the cab so the gasket level it self to the cab let sit for as long as you can before tighten the screws all the way ( at this point if you want to seal the screw head do so... only a littler bit of sealer)

 

Hope this will help anyone who will like to fixed the third brake light
I did on my truck and it work perfect
Due to the hurricane we had my truck was all wet inside cause my light had a leak... the water will stain the head liner and the back carpet will be wet and it may run to the front sides of the truck by the step plate... this happen to me in two hard days off rain... You will know if you have a leak if the inside of truck start smelling weird...

 

First, my thanks to all of you that have helped me out on other threads.
As to the light - I have 2006 F-150 Super Cab. The light leaked badly. I removed the old gasket and replaced it with a product I got at the Home Depot. I found it in the weatherstrip area of the store. MD is the brand. It is called "Replaceable cord weatherstrip" I used 1/8" x 90'.
I tore off enough to go in a circle around the opening on the truck where the light fits in. I gently pushed it into place the same way plumber's putty is installed. I remounted the light and put the screws back in.
That was about 2 months ago. The leak is gone and we have had outrageous rain (Tampa Florida) over that time period.
So, I think we have a winner. The part number is 71548 - it comes in a grey and yellow box.

I hope this helps someone else.

Be well - Chris Moore in Tampa Florida