http://runtime.fivem.net/doc/reference.html#_0x8B7FD87F0DDB421E

Citizen.CreateThread(function()
	while true do
		Citizen.Wait(1)
		local veh = GetVehiclePedIsIn(PlayerPedId(), false)
		if ( IsPedInAnyVehicle(PlayerPedId(), true) ) and ( GetPedInVehicleSeat(veh, false), -1) == PlayerPedId() ) then
			SetVehicleFullbeam(veh, true)
		end
	end
end)

1 minute of research.

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