Light-out indicator
 

1997 C230:

My "light-out" indicator has actuated on my C230. I've checked headlights, tail-lights, turn signals, brake lights... [have not checked backup lights].

Does it show a fault when an interior light fails?

No, it doesn't show a fault with interior lights. If there is a bulb of the incorrect wattage installed somewhere, then the current draw will not be correct and it could make the system think there is a burned out bulb somewhere. But first, check the backup lights to make sure it is not one of those.

Try the elimination process. Turn car off and restart. Light should not eliminate. Turn on parking lights - check indicator. Headlights - check indicator (then high beams). Hit Brakes - check indicator (don't forget 3rd brake light).

lights
 

or the rear side marker lights

In my experience with the "bulb out" warning light, it was always a turn signal. Just replace them, they are cheap!

Third brake light (at rear window)
 

I was going crazy with the same problem checking the rear lights with the trunk open- until our 7 year old asked why the window brake light doesn't come on anymore -- do I feel dumb !