One of my boot lights isn’t working. There is power, the bulb is fine, the problem appears to be the diode. I suspect it has failed. Why is there a diode when the lamp is an ordinary filament bulb that is not polarity sensitive? 
 

If I short the diode the light works. Can I just permanently short the diode?

Are you sure it's a diode? That arrangement may be to slightly reduce the voltage to the lamp using the forward voltage - these are more often seen on a coil connection to prevent inductive surge on de-energising. Are you able to test for continuity in either direction? 

Hi. Yes it is a diode. I removed it from the light unit earlier and tested it. With test leads clipped to the wires it allows current to flow in one direction only (as it should) but wiggling the wires breaks contact, so it has ibviously broken down internally. 
 

It is a 1N4007 power diode, £3.36 for 50 on Amazon. I’ll fit a new one tomorrow and hopefully resolve the issue. 

10 minutes ago, thatsjustdandy said:

Hi. Yes it is a diode. I removed it from the light unit earlier and tested it. With test leads clipped to the wires it allows current to flow in one direction only (as it should) but wiggling the wires breaks contact, so it has ibviously broken down internally. 
 

It is a 1N4007 power diode, £3.36 for 50 on Amazon. I’ll fit a new one tomorrow and hopefully resolve the issue. 

Flyback diode was the term I couldn't recall when I first responded to your message. Perhaps it's there to limit ripple.

New diode fitted, light working normally. Hurrah!