Glowplug circuits - how do they work??? Are they fused....if so where?

Reason I ask is for some bizarre reason the glowplug light isnt illuminating on my girlfriends 306TD (1997) - it isnt just the bulb blown (unless this breaks the circuit) because it has real problems starting & takes ages to finally fire.

Could it just be a case of a faulty glowplug shorting something out?

Any help much appreciated!!!