Since I haven’t sprung for a custom bike with wiring guides or internal cable routing, I’ve had to improvise on my taillight wiring schemes.  The first few times I just strung the wire along the down tube, chain stay, and up the fender stay to the light.  I fastened it with zip-ties.  This was fairly clean from the drive side view, but resulted in some zip-tie mess on the non-drive side.  The last few times I’ve wired up rear lights, I’ve managed to put them under the fenders, eliminating the mess from the down tube. The key has been aluminum tape and careful fender drilling.  Someday, I may get up the courage to drill into the fork or frame, but for now, I’ll play it safe.

https://flic.kr/p/mRi2Y6

You can see the white bike in the background has a similar setup, but instead of routing the wire up the fork leg with zip-ties, I glued shrink tubing on the inside of the fork leg, and the inside of the front rack.  This is used to guide the wire without the use of tape.  Note – on the white bike, the electrical tape I’m using to fasten the wire to the down tube is starting to peel off a bit after a few hundred very wet miles.  It may be replaced soon.  That is one area that the zip-ties win – I haven’t had one come loose yet.  I’ve even taken to loosely tying the wire to the derailleur cable.  Only an issue when you change cables, and the small zip-ties are pretty unobtrusive.