1. The light/horn connector should connect to the four unterminated wires coming from the junction block on the main Control Harness:
One of my Magic Pie Harnesses was equipped with a three pin connector which was presumably meant to connect directly to a battery voltage headlamp/horn unit
(which I don't have) with a matching three pin connector.
The Red Lighting feed wire is connected to one of the outer pins, the Yellow Horn feed wire is connected to the middle pin and the two Black and Blue Ground wires are both connected to the remaining outer pin.
2. If your existing frame does not have mountings for a disc brake caliper you may be able to use
a bolt on adapter kit to mount the caliper to your frame.
I have not tried any of these kits and I'm not sure how suitable they would be for your particular frame or for use with a 14mm diameter wheel axle.
If you have a steel frame
(or forks) and want to try and fabricate some steel mounts to weld/braze onto your frame/forks, then these drawings may help:
3. I would use either electrical insulation tape or nylon zip ties to secure the brake cable
(or hydraulic pipe) to the frame.
Alan