Third hight mount center stop light water in it

[Museau]

I notice water fogging and drips into the high mount center stop lens
First how to acces this ?
how to seal this thing?

The third stoop light located on the trunck, is generating fog, or water drips, inside the lens, we can see water drips in the fixture, i am, concern about it, and questionning my self, if any rust will damage something, or how to stop this problem, ????