rain come in either around the rear brake light or the window. However, the vehicle has a little overhang/visor over the window. I do not see how rain could get in there. It seems to come from the brake light, run the channel around the rear of the truck and down the channel inside the rear door.
If it was removed to change the bulbs, it probably leaks, the gasket/seal is not re-usable.
There are other brands and places to purchase these, this was first one that popped up