A popular modification to these bikes was to add 2 discs up front, then remove the switch wires so you could grab a handful of brakes without a brakelight so the cop doesn't see you slowing quickly after the stoplight race, until you hit the rear brake...Looks like you got one of those....
The headlight, if it has any original wires, contained a pair that ties directly across the rear brakes light switch. They run the same circuit, so just extend the rear brake wires, if your originals are missing, and tie into the pressure switch on the front. Polarity makes no difference.