I dunno why it happens just how to fix it, maybe you should investigate the original fault, I was going through a bulb a week but the LED one is still going, must be a jap thing, my dads shogun does the same.
Does it blow straight away? If so I would get another bulb and test across the battery using a piece of wire. If it blows straight away that way then it's the bulbs, if not a possible short somewhere I guess.
Hmmmmm Interesting. Well it is a new one on me. Is there any other description on the bulb like a manufacturer mark?
I'm planning on taking my interior lightout anyway and replacing it with a 14" Blue Speaker ring which will be submerged between the roof liner and the roof itself. The speaker Ring comes with a control box so it can be either ON > MUSIC > OFF.:laugh:
Usually they blow as they get too hot as you have put a bulb of a higher rated power than you should have in the light holder. Quite often its not actually the bulb failing but the connectors on each end.