Warning light help
 

I have a 420 sel that was passed down from my grandad and all that came with it was the service manual but no owners manual or anything else.

Whenever I start it an indicator light comes on that looks like an upside down light bulb with an exclamation point in the middle with spikes coming off the sides of the bulb. What does the light mean? This is the only light I cant figure out, the rest are self explanatory or visually interpretive.

Re; Warning Light
 

The light indicates that an external light has failed.