Originally Posted by niknak

For the rear light a hole is normally drilled in the down tube near the head tube and on the underside of the bottom bracket shell. The cable pops out of the BB shell and is then fastened on the inside of the fender until it reaches the light.

For the front light, you can run the cable from the vent hole near the dropout to the vent hole near the fork crown and then run the cable along the inside of the fender until you get to the light. It depends however on where you'll be positioning the front light.