Switch on the master cylinder in between the two steel brake line ports is a differential pressure switch.
ABS modulator should be approximately amidships, behind the water tank & between frame rails.
Brake switch should be located below the driver floor, you should be able to see it by looking up into the space ahead of the front tire and behind the steering gear. Should have a black rubber pad that the brake linkage pushes against when at rest, and releases when brake is pushed to connect the brake light circuit. It'll look like post# 2 in this link