DON'T DO IT!
I wouldn't go near the headlight wiring if you have the LED light. Even if you did you would still need a relay but for all I know it could have permanent 12v and the ecu tell the light to turn off and on.
Using the rear light for a trigger is an idea bit if it was me I wouldn't use the aux socket as the main power. I would come straight off the battery via a fuse for the main power and say your rear tail light as the trigger.
Note I don't really know the wc bike and although the older bikes were LED tail lights they were wired like a normal bulb tail light. BMW could have done something crazy like the front light. I doubt it but it's would be worth checking before you start cutting into the wiring.
As Marki says.
When I first got my LC, extra light was top of the priority list.
I did some careful digging, and though BMW would probably deny it, bikes with the LED headlights are deliberately designed to make fitment of non-BMW aftermarket aux lights as difficult as possible.
There are either 4 or 5 wires running from the main harness to the headlight control unit. At least one is an earth, at least two are ignition-switched 12V+, and the remaining wire(s) are LINBUS signal wires. There is no wire you can splice into that actuates the main-beam LEDs with 12V current that's either 'on' or 'off'.
Also, the LED light unit itself is sealed. Opening it up will invalidate your warranty.
There is an American firm that makes a plug-in spotlight controller for BMW LINBUS+CANBUS systems, allowing you to control spotlights any way you please (apparently, it can also be programmed to remove the 0.5-second delay between flicking the switch, and your brights/horn activating). I sent them a mail some time ago, but I can't find it, or remember the firm's name.
