Now that I checked, both of mine are burnt out. Guess now I'm off to the store to replace them tomorrow! I've never noticed them since the light from the other bulbs seem to do a good enough job illuminating the side markers themselves!
Perhaps this should be added to the common problems section ...I saw 2 more Bonnevilles today, niether of them had working markers. I wanted to tell one of the drivers that they were out, but some people get so offended when you tell them something is wrong with their car, even when your driving the exact same thing.