This is an easy way to add light to your bike, so when your riding at night you don't get smoked by a car(If this actually saves your life, take the time to tell me =D). Also putting LED's in anything is fun, so why not?
Step 1: Materials
~LED, the brighter the better
~Battery pack with switch, or other smallish 3-3.6volt power supply
~Drill with 3/16" bit. Use 3/16" for a 5mm LED
Step 2: Drill the Hole
Drill a hole in the reflector right where the clear and the black part of the reflector meet. Go slowly or else the plastic will crack.
Sorry the pictures are all blurry
Step 3: Insert the LED
Take your LED and gently push it into the whole. If you used a 3/16" bit and a 5mm LED you'll have to push a bit.
Step 4: Soldering
Now solder the anode and cathode to the wire, then solder the other ends of the wire to the positive and negative wires of the battery case/power supply. If it doesn't work make sure you got the longer wire from the LED to the positive wire of the battery case/power supply. Shrink tubing would be nice but Radio Shacks too expensive, so I used electrical tape which works just great.
I made this project a looonnngg time ago, so I didn't really think about adding a resistor. Now I would put a smallish value (51-100 ohms) resistor in series just to be safe, but you could probably skip one and be fine.
Step 5: Your Done!
Now stick the whole thing back on your bike, and go for a midnight ride! You can also add the back reflector by doing the same thing and connecting it in parallel with whatever power supply you used.