Here is what I have learned about the rear fog light on these early 1970s cars and why there may be variation in how they are connected and switched.
Although the first rear fog lamp was introduced in Germany in 1966 by Hella, the United Nations Economic Commission for Europe, abbreviated ECE, only issued approval for European-wide use of rear fog lamps in 1978 with regulation ECE R38. Rear fog lamps did not become mandatory in Europe until 1991 under regulation ECE R48. In the US they are still not mandatory.
Unlike the front fog lamps, which should already be used as soon as visibility is poor, the rear fog light should only be switched on when visibility is less than 50 meters. I read this as a driver being able to manually control the switch for the rear fog light, not one that is hooked to the existing driving light switch or front fog light switch as in our early car era the switches were primitive. The reason is that the rear fog light emits 30 times more light than the rear running lights and so when visibility is better, it has the effect of blinding the drivers behind.
So as in life, hook up as you see fit. Just keep it legal