750 K3 Horn gets 12 V from a black wire (switched from ignition switch)
The horn button grounds the other horn connection through a LG (Light Green) wire. Beep!
The original bike had a hydraulic pressure switch on the lower fork tree. You should find G/Y and Bk wires near that location. When connected, the brake light should come on. (when ignition switch is also on)
Cheers,