Hi Ted,
It was an Amphicar oversight not to have some form of gasket there since water is held behind the reflector, which rusts the reflector and the back of the car. What I did was use a rather thick rubber washer behind the reflector so that the reflector is spaced out about 3/32" from the body. That way water that gets in there will simply run out and not cause a problem. Also the flat rubber washer (available at most any hardware store) acts as a seal for the mounting shaft. It works well. I've used that system for over 6 years now with no problem.
John Friese
67 Whie
67 Red