OMG, you've had that car how long and haven't pulled the cluster yet? I thought it was a mandatory first job for all new W123 owners.
You could replace the rest of the bulbs while you're in there. But once you find out how easy the cluster is to pull, you won't be afraid to pull it again later. Hope that made sense. Anyway, the little bulbs are some wacky euro size and if you just buy replacements at the parts store without being careful, you'll probably get the wrong ones. I've usually found the euro bulbs at the bottom of the display in packages with a green background. BTW, you could replace the main light bulbs with overwattage ones, but they will melt the enclosures. Don't do that.