I had the same problem until I fixed it recently. I even drove around with the backup bulbs removed because I didn't want them to short out from the puddles of water in the housing.
Although you could have found the solution from doing a search in the forums this is what you have to do. Remove the entire tail light assembly (about 4 nuts and a couple of electrical connections). Then put a few strips of something like
3M Flexible Caulk all along the rubber gasket. You'll sort of form a big rectangle. You can order it from that website but I found some at a local auto parts store.
Some people will say you only need to apply the caulk along the top portion but since the original rubber gasket goes all the way around I went all the way around it too. Since the 3M kit has 20 yards of the stuff I also put it across the entire top edge of the tail light assembly. Don't get it too close to the edge so it doesn't show when you put it back together.
It's been about a month since I did mine and a few heavy rains and the backup lights are still nice and dry. There are other trunk leak issues people have been having with the vents but yours seems to be the gasket area around the lights like mine was.
