I run mine mine a variety of ways depending on the bike. Some I use small black zip ties and run it along a cable. Others along the fender stays and then along the cables. On at least one I ran it inside the fender and held it in place with dabs of silicone caulk. I have never run it inside a frame on a retro fit. I have two bikes where the fenders are actually the conductor, they use a snap as a connector. On one of those bikes I do believe the wiring may be inside the frame, will have to double check.

Aaron