You didn't mention what kind of car you have, but my '64 Belvedere has several flat plastic straps that attach to holes in the radiator support and hold the harness up. If you look thru Year One's website in the electrical section (they're under "wiring strap kits"), they have a few pictures.

That's just on the horizontal area of the radiator support though. On either end, the harness just drops down and feeds into the area where the headlights are. I think I might have one welded on bendable "clamp" in the area behind the headlights.