[JayJay]

To supplement what nk suggests, I'd really concentrate on the ground side. Horn and lights operate through two separate ground paths. Hot to the horn goes through the steering column, the horn button and eventually through the light switch to ground on the frame. The lights go through the light switch to the lights, then are grounded through the connection to the light bar and from there to the frame. The fact that you are having trouble with both suggests a bad connection between the ground side of the battery and the frame.

How does your brake light work? Again, hot to the switch (which is open when the brake pedal is at rest), then through the bulb to the frame.