Here's a really cheap brakelight monitor:
Take the cover off the third brakelight and drill a very small hole in the top surface immediately above the lamp filament. On the inner surface, glue on a piece of plastic, either clear or a color of your choice.
Now, every time you step on the brake, in your rear view mirror you will see a small bright spot in the middle of the rear window.
Granted, this only will tell you that the third lamp is working and not if one of the primary lamps is out. But, if you see a light in the rear window, you know that you have at least one brake light showing.
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