These festoon bulbs do get quite hot, and while a bulb may look as if it has a complete filament, often the soldered ends of the bulb that push into the fingered holder on the car may look ok, the soldered ends on these often melt away and the bulb fails. So worth either substituting the bulb with the one that works, or fit a new pair. The other area where things go wrong is the circuit board itself has known issues too. The contact on the board for the bulb holder were made with a process that has proven to be a bit unreliable, they are kind of riveted direct to the pcb track. After a few bulbs have been changed, the contact to the pcb becomes compromised resulting in what is considered a bulb failure, but in fact is the holder and pcb design that has let go. The way I have repaired these during my interest in Rovers/cars in general was to remove the whole pcb, clean everything thoroughly including the doublesided take holding it to the car, Solder the finger/bulb holders to the riveted point and a small puddle to contact the board as well. Upon testing and good result, apply a pcb lacquer to the underside of the board allow to dry and apply a good double sided tape to it prior to fitting to the vehicle. Time wise, I would allow about an hour for this job, and the use of (Leaded Solder) is far better as it has a lower heat requirement than unleaded and works far better.
Hope this helps and good luck.
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