The gas filler bowl gasket was not sealing and water would leak into the fender area:



To remove the assembly you must diconnect the actuator from inside the fender cavity. The plunger and mating point has a locking tab that you can see by my thumb. It slides apart easily when you do it right. I was wedged in the trunk with my hands out of view when I removed it, so I didn't actually see what happened.



After the plunger is removed you must remove the latch thing. It is another task you must do by feel rather than sight. The picture shows it already out of the car. You must do it from inside the trunk.



Here's another view to clairify what you can't see:



The filler assembly is held in place by 4 tabs. I wish I had a picture for you, but I don't. The tabs are accessed from inside the fender area (naturally), and you will have to find them by feel. There are two on top and two on bottom, and you can depress them with a hard edge like the blade of a flat screw driver. They are a real pain to get to. Also, get a helper on the outside to pull on the assembly as you work your way around the tabs so they do not snap back into place before you get all four done. The door hinge extends behind the fender and makes the removal more difficult. Protect the fender with something or you will scuff the paint as the filler door grinds against it. I used a large refrigerator magnet, but you could use a few layers of tape, too.

A bead of black silicone does the trick. Just reinstall the filler housing and wipe away the excess.