I'm going to assume we're talking about stock light bulbs, and not LEDs.
I'm a firm believer in examining all the wires and contacts in a circuit. Being thorough isn't a waste of time.
- It would be utterly usual for contacts on a bike this old to be so sloppy. Loose female contacts can get sloppy, and the wires get disconnected while simply closing up the headlight bucket.
- Could be a small cut in any of those wires, especially under the seat.
- The rear winker wires go through the fender and meet up under the seat. Are any of them damaged?
- Corroded or dirty contacts at any of the wire ends need to be cleaned with a bit of Scotchbrite, etc.
- BrettP mentioned the switch. Could be a coccoon up in there. (That's not a joke)
- Corroded bulb sockets?
- 77' rear winker ground wires are soldered to the steel posts which hold the winkers on. Is the solder in tact?
- Correct bulbs for the correct signal? 77' has front running lights along with the signal, so double filament.