You mean the switch that turns on the neutral light?. The method varies but it's usually a contact inserted into the gearbox that rotates with the gear selector cam drum. On Hondas it can be found next to the gearbox sprocket. It looks like a small black plastic thing and has a single wire going to it. The lamp receives 12 volts at one end and only comes on when that switch earths the other end of the wire.