Dear Brian
Any Mercedes w113 can be retrofitted with a hazard lights relay with the correct knowledge without modification of the main wiring harness.
Early w113 were fitted with a hazard relay switch which didn’t have an illuminated knob switch when active.
You can read more about it on this thread
https://www.sl113.org/wiki/Electrical/HazardWarningLightsIf you are handy with electricity and reading wiring diagrams, you can figure out how to build such a harness just by looking carefully at the technical pagoda manual on the wiring diagrams section. Also will need the right components which are somehow hard to source (I mean the connectors 8, 12 pin and terminals).
The only thing I need to know to built a sub wiring harness like this for your particular car is the kind of steering column multifunction switch your car is fitted with.
There are two types. One with 8 pins and some more modern ones with 12 pins. Generally speaking cars with foot operated high/low beam headlights switch have 8 pins on that steering column switch. Cars which make the headlight high/low change on the steering column (I am not talking about the high beam light horn) have 12 pins.
Just look below the steering column and see how many pins has the connector coming from the column. And take a picture for me of the whole thing.
In that way I will built from scratch the right sub wiring harness, which will be fitted like a plug and play solution on your car.
As your car is year 1969, the most probably answer is that the connector has 12 pins.
Best regards
L.peterssen