Achim,
Thanks for your very helpful suggestions. You are correct about the wire being behind the insulation pad, but I was able to get just enough slack to access to the three screws. I happen to have a wonderful, stress-saving small screwdriver that has two spring-loaded lugs in it that can hold a screwhead tightly while it's being threaded and that was my salvation. I'm no longer dazzling oncoming drivers at night!
Lowell