Why does the horn blow when you brake and turn?

Why does the horn blow when you brake and turn?
The problem here sounds like a short-to-voltage between the horn circuit and the brake or other circuits. This essentially means that when one circuit is powered on, such as your brake light circuit when you depress the brake pedal, an additional circuit is also powered on by that circuit due to the copper wiring from the two circuits touching one another. If you are familiar with a DMM (digital multimeter) you can diagnose your wiring to find the source of the problem. Otherwise, you might be taking the vehicle to a shop to have it diagnosed/repaired. I would recommend an automotive technician that is ASE certified specifically in automotive electronics, not just any regular "mechanic."