in the late 70s i had an occasion to drive a lady's Lincoln and noticed inside the rear window was a box with two little lights. i figured out what the ywere.
on a recent post i saw where someone installed a blue led in the seatbelt bulge so when he hit the brakes the lite would come on and light up the inside rear of the car....to each his own.
my idea is to mount an led somewhere inside the rear to be seen in the rear view mirror. when the led comes on it means the brake light is working (splice it in series). so many people drive around not knowing a brake light was out.