Unlikely to be an earth issue if hazards work.
I think on a model of this vintage, there maybe separate hazard and flasher relays, so could be the flasher relay faulty?
Also, if you have a multimeter, check if you have a supply to the common point on the indicator switch.
If the GoMog diagram is accurate, then they changed the wiring layout between 1983 and 1986, as I have a single relay. With my 1986 car in normal operation, the indicators earth through the other side. At the rear, there's a connector that links both sides, which can fail to prevent one or the other or both sides working. Because the connectors are quite exposed, it is easy for wet and corrosion to get in there.
Years ago I had an issue that if I went over a particularly savage bump in the wet, I would lose RH indicator although everything else worked fine. I ended up completely remaking the earth connections in this area, introducing addition direct earths for each side.
