This is tractor wiring diagram. The tractor wired here is Ferguson TEA20 Tractor. The wiring is colorful. The wiring diagram begin from its generator to relay LUCAS, charge indicator 12V incandescent, Ignition key switch, fuse 17A to supply to lights switch, and more. Auxiliary equipment such as headlights, taillights, reversing/work-lamp, horn etc. are not shown, but would be wired as for negative earth, between chassis/battery +ve terminal (earth) and either starter switch -ve input (independent of ignition key), or via a relay fed from starter switch -ve terminal but switched via output of ignition key switch (dependent on ignition switch). Original TEA horn button has one terminal which is shorted to switch chassis and ground when pressed, therefore to use this button the horn must be permanently fed with 12v and grounded via the switch. Remember that the tractor is positive ground! For modern single pole button with isolated terminals feed live to one horn terminal via switch and ground the other horn terminal. Full ferguson tractor wiring diagram here.