I don't know if this sounds obvious but you should be able to establish which bulb or set of bulbs the failed/failing one is in. I'm sure someone else has posted something similar to this.
With the engine running (beware the reverse gear check, best tested with the engine stopped although the dash light will be permanently lit).
Turn dial once and sidelights go on; failed? It's one of 6 (possibly 8) bulbs, sidelights front (2), rear lamps (2 or 4) or licence plate lamps (2)
Turn dial to next level, headlamps; failed? It's a headlamp bulb (2)
push indicator lever forwards; failed? Beam (2)
Pull dial out once; failed? front fogs (2)
once more; failed? rear fog lamp (usually just the one although there is probably a second bulb inactive in the opposite housing)
Indicate either direction; failed (if not obvious at 2x the speed
) then it's probably a wing repeater (2 total)
Touch brake pedal; failed? brake lamps, this usually stays on even if you take your foot off the brake (2 lamps, sometimes three but the later LED cluster may not register)
select reverse gear; failed? reverse bulb (2).
I think that's it although I'm prepared to be told where I'm wrong