Hi,I have 1987 ETZ300,the rear brakelight switch on mine is a tubular type and is fitted on a fork on the rear of the battery box.A long wire and spring is connected to a small lug on the brake pedal pivot.Mine also has a conection on the rear hub,but has a plug in it,and this is how it was when I bought it.
If yours doesn’t have any of these,I have seen other MZs at rallies that have the rear stoplight switch on the left side swinging arm on a bracket close to the rear brake cross shaft,with a spring connecting it to the arm of the cross shaft.
Hope you find this helpful. Terry.