Bad regulator or just install new alternator (voltage across battery terminals with engine running should read about 13.5 to 14.5 volts). Because if you ignore this you'll likely burn out your bulbs, creating bigger issues !!! This actually happened on my 1979 Toyota truck maybe in mid-1980s, all bulbs except headlights and the (fortunately) on-off nature of the turn signals/hazard lights got me home.
