You likely have an 80's GM Car, Truck, Motorhome, or Semi-Truck and this requires a different wiring configuration on the low beam headlight plugs.On your two low beam headlights only, there are 3 wires that go into your headlight plug. A low beam wire, a high beam wire, and a ground wire. On both low beam headlights, you want to swap the low beam wire with the ground wire.
If that does not fix it then put it back and then swap the low beam and high beam wires.If you have a test light to see which terminals on your low beam plug are low beam, high beam and ground and then mark them. Then go the headlight and identify which terminals on the light are low beam, high beam and ground so you can them match them accordingly with your headlight plug on the vehicle.