Very likely a failed contact or wire in the front swivel indicator assembly or very possibly detached wires in the indicator switch on the dash. If its the original type it could simply be wires snapping from the block or terminals.

This wiring is at an age where both the wire and the insulation has become very brittle and it does not take much for it to come loose or snap. Just engine vibration alone can really cause a lot of problems with the cab area wiring.

A few good cures. Heat shrink sleeving on multiple connecting wires helps reduce disturbance and bolsters the insulation.

A light spray of Jenolite Electrical contact cleaner on all exposed connections and a spray of silicone and plastic lubricant on all the wiring minimises the brittle wiring and plastic components and a small spray of WD 40 onto damp prone wires helps keep the corrosion at bay and lubricates.