Making the CHMSL flash does not make driving any safer for you, if that is why you are doing it. There are copious studies that show that flashing does not help, because it takes too long for a filament lamp to flash enough time for the following driver to understand what is going on. Also, even if you got a following driver that was actually paying attention, there are other valid studies that show that even in obvious emergency situations, drivers rarely apply the brakes hard enough to avoid a crash. That is why M-B, Toyota, BMW and others install "brake assist" on all their vehicles. It senses when a driver has quickly remove pressure on the acc. pedal, and moved to the brake, and how much brake pedal pressure has been applied. It the vehicle determines that you are not pushing hard enough, it pushes for you, to protect itself from you.
So don't imagine that that you are doing something worthwhile.
However, if you are doing it for a trailer queen, okay, just remove the rear plate and reach in as stated above!