When I swapped out all the lights for LED on my Cobra in 'O5, I added a tailfin mounted LED brake light. I used a 12 volt relay for the "logic", driving the relay coil with one brake light and the relay NO contact with the other. Both brake lights on at the same time, fin light on also.

There's another inadvertent mode also. If you have a turn signal and brake lights on at the same time, the fin light flashes with the turn signal :c(

I have the aluminum top and had to mount the fin LED with wedge to bring the LED beam angle down enough to be effective for following cars. My center marker lights flash with turn signals, don't remember if it came that way - I may have changed that too.