Need to check your tail light and back brake bulb sockets. Clean or replace both. This will fix your problem. If your bulbs blow and break you have water setting in the lens. After years of baking in the sun the plastic cracks then water gets in and sits inside of light housing. When you step on the brakes water hits the hot bulb causing the bulb to break and blow. The SWISS need to start using GLASS instead of plastic. But they would not sell as many parts if they did. . .
Oh, I found out why the use plastic headlamps instead of glass like in Europe. Because of U.S. DOT. The plastic lens give a glow of light to reflect on our roadsigns where overseas they have lighted roadsigns and the lamps do not glow but shoot a direct ray of light. ;(