John,
It has a separate small guage insulated wire that you plug into the modulator unit 'inside' the headlight bucket. That wire has a waterproof photo-cell sensor on the end of it in a brass ring. You just run that wire through the back of the headlight housing gromet and attach it (with a provided wire wrap loop) to something around or near the top of the headlight housing like a brake cable etc. It just needs to be where it faces upwards and unobstucted so sunlight can get to it. When it is 'daylight' the light sensor lets your modulator work when you switch your lights to high beam. If you switch to low beam the modulator goes off.
Of course when it is dark enough outside the photo-sensor 'disables' the modulator and your bright beams work as normal. In the USA the DOT doesn't allow modulators to be used at night.
ps- The photo-cell can be mounted flush in the top of your headlight housing but you would need to drill it.
Bob
