On my 1991 560 SEL, the illumination on 2 switches does not work. They are the rear defrost switch and rear dome light. All other switches illuminate fine. I replaced the switches thinking they had burned out bulbs but they still don't work. I noticed that both switchjes have what looks like a fibre optic cable which probably provides they illumination. Question is, where is the light source(s) located? Anybody know?