Get Gorilla 100% Silicone clear sealant. The company site states that it is OK to use on auto, metal, and plastic. I did a thin bead and spread it around with my finger around the shark fin. With it being clear you can't notice it. Also, it is not going to yellow or crack so that is a benefit.
While at it, go ahead and do the same for the top and both of the sides of third brake light. I have seen people state theirs is leaking there as well.
I did this quite a few months ago and have since taken my 15 silverado through a touchless car with 0 leaking issues which it did leak before.