This instructable relies on an off-the-shelf AA minimag and LED module, brand name "NIGHT-IZE." It was about $12. It also relies on several serendipitous brackets and clips to mount this stuff to my bike. Readers can search out the exact stuff I used, but I figure the average Instructables reader is resourceful enough to figure out their own version.
Other materials/tools: miniature momentary and toggle switches, 10" lengths of 24 AWG wire, some heat shrink, a 1/8" female phone jack and plug. I used a TRS (stereo) plug but it doesn't have to be. In fact some other kind of jack/plug combination might work. Oh, and the spring from a retractable ballpoint pen!
The LED module is not terribly bright - about the same as a 3-LED head-worn lamp. Don't bother comparing it to some $150 HID paint-peeler. It is enough to to see the path in the woods or the crusties lurking under that bridge. My night-ride philosophophy is stealth. As someone else on this site said, "they can't aim for what they can't see." My system uses a momentary and a toggle switch in parrallel so I can have the light on all the time or only when I pull the trigger.
Step 1: Mod the flashlight tailcap
I don't know the threads on my jack, but I compared them to my tap and die set and decided 5/16-24 would be close enough. A pilot hole was drilled, enlarged to 17/64", and tapped.