Have a look at gomog wiring diagram download
here. As suggested hazard warning lights for obvious reason as well as horns of that period were traditionally wired direct to battery and not even through ammeter, so would look at ignition feeds to and from switch. You will see the switch feeds various white wires when in the on (run) position. I suspect one or other of these are not connected or possibly switch's main feed from ammeter (thick brown/white) missing, could even be a faulty switch. Suggest checking all your dash wiring against this diagram before connecting battery.
