Mercedes Benz repair: Mercedes Benz 200E (Year 1989), mercedes benz 200e, indicator bulbs

Question
Headlamp light not strong and not enough illumination at night
and have to use the fog light. But able to use main beam and fog
lamps. Indicator for 'exterior light bulbs has failed' also lit up
when switching headlamp on. However, when I turn to main
beam, the indicator gone.

Answer
Never remove battery cables from the battery with the engine running, it will destroy the voltage regulator.

Replace all Failed indicator bulbs.
The alternator indicator bulb is a controlling part of the charging circuit = the system will not charge without a good bulb.

Inspect ground wires to lights.
Remove battery to body ground cable, and body to engine ground cable, clean cable + mounting area, then reinstall both cables.
Inspect alternator belt, is it loose, cracked or glazed = replace with new as needed.
Test charge voltage = at least 14.5 volts, any less = replace voltage regulator.