Recommendations for a waterproof sealant for exterior

I'm planning to replace my third brake light soon, and after hearing about the recall and some of the horror stories of water getting into the roof, I'm not looking to take any chances. I need some recommendations for a waterproof sealant that is safe to use on the exterior of the truck, and

I've checked out some of the rev sealants like permatex, loctite, and gorilla, but some people say they don't hold up well in the sun/heat over time. Since this will be on the roof and exposed to the sun at all times, it's pretty important that it holds up.

Anyone have any experience with these (or others), and can vouch for water-tightness and resilience to UV rays?

 

Sikaflex 221

 

Mulco clear sealant.