Well if you have high beams on all 4 headlamps then your grounds are good.
If not check the grounds for those lamps...
If all 4 light in the high beam position then you need to check the high beam switch on the drivers floor . You can dissconnect it and jump the violet to the green , then turn on the headlight switch if the low beams light then your floor switch is bad.
If they still dont light You will have to chase the violet wires out to the headlights .
You can also disconnect the headlight connector at the drivers headlight and see if you have 12vdc from violet to chassis ground when headlight switch is on and floor switch is in the low position
Im guessing floor switch . Or bad connection at the first headlight , corroded etc......
The lowbeam circuit goes from the headlight switch to the floor switch then to the firwall bulkhead fitting to the drivers outside headlight so fairly simple to chase