1994 Toyota Truck When I turn on the lights the voltage drops

1994 Toyota Truck mileage: 88,000. When I turn on the lights the voltage drops so low that you can feel it. It returns to normal relatively immediately. The blinker has always turned itself off when you start turning not after the turn is completed. When I have the blinker on you can see the voltage gauge move in unison with the blinker lights. I helped someone start their car with jumper cables and mistakenly attached negative to positive. Alternator tests fine. Thanks