If the bulbs are of the dual filament type its possible that one of the filaments is burnt out,,i.e brake filament illuminates,,running filament does not,,try replacing the bulbs.
If bulbs are single filament types,,i.e. 1 set of lamps for brake lights and 1 set of lamps for running lights, then you will need to use a volt/ohm meter or test light to determine if there is voltage present at bulb socket,,then connectors, then wire harness/ bulkhead connectors all the way up to headlamp switch to determine where voltage loss is present at.
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