I found this thread very helpful.
The switch seemed to fit best on the clutch lever master cylinder mounting bolt on the left clip-on on my 312R. Spliced into the fan relay black/white wire and cable tied a wire up to the switch with enough slack for handlebar movement. I then just ran a short wire from the switch to the same clutch lever master cylinder mounting bolt and also used it as the earth. Maybe not the best earth but it is simple and seems to work.
Manual Fan Switch Install Write-up! Disclaimer: The work shown here isn't exactly what I would call "professional" haha. I'm going to go back through and button everything up with heat shielding, etc. I wanted to see how it affected the bike before I put too much effort into it. I couldn't...