You said that you didn't understand why your series clearance lights all failed so I gave you a possible reason.
Well, the lights are all the same model by the same manufacturer, so in theory, it should work just fine. In reality, though, they worked for a few months, and then started burning out. Not many have survived, and I have not been able to ascertain if it's due to the series wiring somehow, or if it's just that the lights are junk.
I'm going to replace them with more of the same, but this time I'll run them off the 12 volt output from my toad converter. Will see how long they last.
BTW: Does your bus indicate when you have a bulb failure on the brake and/or signal lights? That may be an option but I thought it was standard on MCI from way back.
Not that I'm aware of. If so, this is the first time I've heard of such a thing on any MCI. There is one indicator light in my panel that is not marked, and I can't figure out what it is, but it's not lit, so I guess it's a possibility. Anyone else ever hear of this?