I have put extra red bullets on two bikes as brake lights, basically as said above to act like the 3rd break lights on cars.
A problem with the combined tail/brake light is that there is no indication that you are breaking if someone looks away before you brake and back after you have braked.
On the one with LED tail cluster, I have put the modulator on the original brake light with the 30% brighter option, but wired the bullets in to the loom Before the modulator so they are normally off, but come on unmodulated with the brake light.
Thus the normal tail light is brighter, the normal brake light modulates initially, and the two bullets are plain additional unmodulated brake lights.