[byroncobra]

Stumpy
I was scratching my head over the switch as well, on all the time, until i realised the 2 poles are connected inside.
The solution........the switch earths through the mastercylinder, so you only need a single wire returning to the dash lamp, ie 12v ign supply to the lamp, then the earth running from other side of lamp to either pole on the switch. When the plunger is pushed in by the piston (as per Gary's eloquent description) it completes the circuit and on goes the lamp.