Steve, great job on the tech info, I just did the same for my u20 powered 1600. I even put in an adjustable thermostst control on mine. I wired into the ignition voltage and the battery
I used to have the rocker switch with the light and I would notice at night that with the fan switch off the little light would come on due to the fan turning at highway speed. It must have created it's own lowvoltage output.. Sounds far fetched but that is the only explanation I could have for the light coming on with the switch off... Bill