Well if only one light went out on the dash and the rest of it still works, I would say it's a bulb. If it was a fuse, then I would assume the whole dash would go out. Since the W124 dashes are similar to my 126's, you have to pull the instrument cluster to replace the bulb. Both my bulbs are attached to the speedometer unit, so if yours is the same, unscrew the speedo unit, pop it out and the bulbs are right there inside the plastic housing. Hope this helps.
'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.
"A Mercedes-Benz is like a fine wine, it only gets better with age."