for the sake of discussion, let me offer some photos of my commuting bike.
It's not tidy in the way that some rando bikes are, but perhaps it might show some options or ideas.
Let's discuss this bit by bit... and start at the fork. The bike uses a dyno hub, and the light attaches to the top of the fork blade, so there is a need to route wires from the bottom to the top of the fork blade. I relied primarily on the braze-on fittings for the fender mounts and the rack mounts as places to attach the wires.
I should probably note that I use a set of pins and sockets to make an easy disconnect for the Schmidt dynamo...
I have a dynamo powered tail light and run these wires along the top tube. The frame is custom, so I had the option of coming up with a really clever idea for routing the wires. The frame is also S&S coupled, so the method had to accomodate breaking the frame in half.
The result is a series of simple loops that I can tie the wires to.
Not particularly clever or clean, but functional and has kept the wires out of the way and out of trouble.
The tail light is mounted to a Tubus rack. I just run the wires along horizontal tube of the rack, securing them with electrical tape. I also use connector pins and sockets to be able to easily disconnect this section of wires from those attached to the top tube of the frame.
The ultimate in tidy wiring is to use the Schmidt SL fitting, IIRC, which allows the wires to be concealed in the fork blade. If the frame is equipped with a small hole on the underside of the top of the down tube, then wires for the tail light can be routed through the down tube and to the end of the chain stay. With another hole in the chain stay, the wires can then exit the frame and be routed to the tail light.
There must be pictures on the web of someone who has done this... maybe Peter Weigle??
Steve in Peoria