When I bought mine, for prior bike, it came with a mounting adaptor that attached to the two bolts on the left handlebar controls. I believe this is still sold but as an option now. What is left hand toggle switch for?
Yes the adapter uses four screws in the back of the controls. The switch is high beam of my Denali set up, this is the original single pole waterproof switch supplied with the D2 Denali , I removed two pins and swapped connections, so low is on when I key on the bike, I have my Osram 65 watt in the high beam so I prefer running the Denali on high and switching the OEM high beam on when I don't have opposing traffic . My over 6 hour ride in the dark, never had 1 car flash their lights with the Denali on high beam. That is a whole different story with the Osram on, much of that night ride I used the passing switch , as my focus was looking for moose and other nocturnal animals, several times it was several seconds facing headlights, just a simple release of the passing switch.