Right, for once have done something I said I would.
So, first the bad news. It's not a connect to ground
The good news, it's even simpler.
Going on the numbering of the connector on the switch PCB (Printed Circuit Board)
(the front bit) there is a common signal pin 7. (ground is actually pin 1, so I was wrong).
For main lighting there are two switches: sidelights (pin 4) and headlights (pin 9). The switches are closed for lights OFF (so 4 and 9 are connected to 7) 4 is disconnected from 7 with 9 still connected for sidelights lights on. 9 is then disconnected for headlights.
The foglights work in a similar but conected briefly to toggle status. 3 to 7 is rear fogs, 5 to 7 is front fogs.
Now I haven't done any wiring to try it out, but my multimeter tells me it works this way. Fogs could be done in paralell to the switches, side / head would need a series relay in the ribbon cable or a cut on the PCB
(though I couldn't spot an easy place to cut).