Ron's third brake light got me to thinking.... I have an extra license plate light fixture around here somewhere, I may just try drilling six holes through the front of it and then mounting six
clear LEDs, in series, through those holes. Those LEDs would be
RED and the superbright type. With a source voltage of 14.6v, and each diode having a diode forward voltage of 2.2v and a forward current of 20mA, an 80 ohm resistor would be all I needed and just feed it directly off the brake light switch. It would be hardly noticable when not lit, and would look something like this when you were on your brakes (although obviously much brighter):
