Usually the headlight circuit requires that both the ignition switch and the headlight switch be turned on in order to for the lights to work. Your lights must have been modified in order for this trouble to occur. Normally the light switch supplies a ground connection for the lights and the ignition switch supplies power to the relays which are also controlled by the light switch. Check to see if power is getting to the low beam lights even though they are off. If you do have power there this would mean there is something going on with the ground connection. There may be a problem with the light switch also.