If you're looking for a switched lead, jumper off the running light in the back. FWIW, I have a relay under the tank that provides current to my heated grips. This way the grips turn off/on with the key. I soldered a wire to the running light in the back as a source. My R/R is connected to the wire coming from the tail light: works great.
I've had the enduralast system on my airhead since 06 or 07. I thought it was straight forward to install, but did have some "learning experiences". Mine had to do with the connections. I was having a weak charge under load. I found my R/R had a diode out, but as I worked through the system, I found the connectors that came with the system were loose. I pulled all those connectors and soldered everything with shrink wrap. It's been good since. As a result, I don't trust those threaded connectors.