the light works on the glow plug circuit, if you have a glow plug not working this will stop the light from working as the is a break in the circuit,some people have the plugs wired up independently so if 1 plug fails the others still work,in your case they are wired up on 1 circuit, there fore you have a plug foooked
Don't know about Series III specifically but all the glowplug wiring I have seen has a common rail to the one side of the plugs with the other side being the earth of each plug. So all the plugs are independant of each other. They can and do burn out but not likely all at once. The power feed usually comes off a relay and the light is fed from the relay output. No light would usually mean no power to the glow plugs. Put a meter on the plugs. If youve got power then one or more of the plugs is faulty. If you haven't then the supply is faulty.
