Hi Tyler,
Having performed a similar modification last month (wiring the flasher function on the multi function switch), I am interested to understand what you are suggesting, which is different from what I think I saw elsewhere on the forum and what I did. Please note that I have almost the same car as Yves, by coincidence: a 1969 289SL initially specs for the US.
My car is being worked on right now so I can’t go and check this sub-harness thing, but I think I recognize the connector in your picture (since to access the multi function connector you have to disassemble the plate on which all these connectors are located, I opened them all, cleaned and sprayed all the contacts, and put them back together).
From memory, all the pins, male and female, were wired on all the connectors, except the female pins nº5 and 8 on the multi function connector. I ended up doing exactly what Yves is suggesting, inserting females pins with new wires soldered on in the multi function connector and running the corresponding wires to the fuse box (fuses 2 and 10).
I’m ready to go in again, remove the extra wires I put in and use the wires in the hazard sub harness, but what I think I saw in there doesn’t match your picture, in that all the pins were wired in that sub harness. On the other hand, pins nº5 and 8 on the female part of the multi function connector were not wired.