if you can get the starter to work jumping the relay then either the yellow power wire to the relay is not getting power or the brown ground wires is not grounding , if it is an auto it grounds at the neutral safety switch in the trans & those plugs can get very hard & intermittent when 40 years old , this would be the most likely problem , often the plug will just work loose , you can buy new harnesses . The last possibility & sencond most likely is the relay itself has a bad contact .