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So, you've replaced the internals so that some sort of 12V lamp will fit into the flashlights? Once that, or something is done to allow 12v to operate the flashlights, it is simple. You already have a wire that is hot when the brakes are applied, Splice another wire to that one. Run one to one of the flash lights, and the other wire to the other light. Do the same for the ground wire.
It becomes a "whole 'nother story" if you're not replacing the flash light guts, or trying to use them as combination tail and brake lights.
7 Surprising Harley-Davidson Products that Are Not Motorcycles
Slideshow: The bar-and-shield logo shows up on far more than motorcycles, some of the company's most unexpected products have nothing to do with riding.
Slideshow: From the troubled AMF years to modern misfires, these bikes earned reputations for reliability issues, questionable engineering, or disappointing performance.
Crazy Bunderbike Build Looks Amazing, But Is It Impossible to Ride?
Slideshow: The Swiss custom shop has taken a Harley Softail and stretched it into something so long and low that it looks closer to a rolling sculpture than a conventional motorcycle.
Engraved Rebellion: Inside Bundnerbike's Glam Rock II
Slideshow: A standard cruiser becomes an intricate metal canvas in the hands of a Swiss custom house known for pushing Harley-Davidson platforms far beyond their factory brief.
Slideshow: Harley-Davidson's challenges aren't abstract; they show up in dropping shipments, shrinking dealer traffic, and strategic decisions that aren't yet translating into growth.