For 78 I think there are 4 breakers on a stock 78 FLH...main circuit breaker off battery + side, lighting, ignition and accessory which were mounted on the underside of the dash plate.
Is the wiring original? Post a pic perhaps?
easiest way to see which breaker operates the lights is to disconnect one of the wires and turn on the switch. Lights will not come on with wire disconnected.