I have a 1998 1100
It should be grounded through the same plug as the power comes through.Power goes in,across the bulb,out the ground to wherever or whatever it is actually grounded to.Is this an intermittent problem?If it is,just kind of gently pull on the wires leading to the plug,about 6 inches away if it is accessible enough,and turn the wires in all kinds of directions.Left and right,up and down,in small circles and see if it affects the light.If the light goes on and off at all,briefly or whatever,and does not stay on,inspect the wires and the connector.The ground should trace back to either a visible central grounding point,or something else that is attached directly to ground.Also,I am not familiar with all bikes/models/years but you could check the bulb itself and see if it is screwed in tight enough or whatever.If it has a central tab for the type of bulb that you push down and twist to insert,that is the power input and it grounds on the sides of the inside where it is plugged in.If the tab is pushed down too far,it may not make good contact with the bottom of the bulb.If you have ny electrical measuring equipment,it would be useful.If not,no big deal,we can figure this out.The light also comes on automatically when you turn the key too right?Let me know if you find anything,if not please feel free to reply and we can figure this out.Good luck and ride on! Greg