I'd agree that replacing the fixture is the easiest solution. When you get the new one, double check the gasket that goes between the lens and the fixture. I had these on my coach and they were problematic and frequently allowed water into the fixture causing corrosion like you have. In the end I cut a new gasket from neoprene to replace the paper or cork one they come with.
1994 Excella 25 Follow the build on Gertie!
1999 Suburban LS 2500 w/7.4L V8
1974 GMC 4108a - Custom Coach Land Cruiser