This question has been asked several times over the last few years on Bikeforums, and the answer is still the same--there's really no good ready-made solution.
The only good thing I've ever heard about any of the manufactured ones was that one used a gravity-operated switch to control the brake lights, which was pretty clever.
You could build something very easily if you wanted; just wire up two rear LED lights left and right, to a pair of NO-button switches beneath both handgrips. You'd just press each button on-off-on-off to make it "blink", and you could hold both while braking for "brake lights" (though the turn and brake lights on cars are different colors, but who cares, I think it would be close enough for people to get the idea).
Alternately you could have amber turn signals and the "gravity-operated" brake lights red, if you could find a switch that would trip the same way.
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