Electric cooling fan wiring question

Did a search, but didn't find exactly what I'm looking for, so I'll ask the experts.

I'm in the process of adding a new 26" core radiator and dual 12" electric fans to the Scamp. Everything is now mounted and I'm ready to start roughing in the wiring. I'm using one of Jegs dual electric fan relay kits for the wiring, and the instructions that came with it are pretty lame. That led me to an internet search for wiring diagrams, which led me to the Champion radiator site, where I found a pretty nice diagram that explained nearly everything well enough that even a rank amateur such as myself could understand it. Everything, that is, except one thing. In the diagram, it show the yellow wire going to the ignition switch, or a stand alone switch, and I want to use the ignition switch method so I don't have to worry about forgetting to turn off the fan switch.

Which leads me to my question - what wire am I looking for to hook in to, and is there a wire that will work under the hood, for ease of wiring and to keep the runs as short as possible?

Many thanks in advance for the guidance.