Hi,
On my 84 HPE it is the grey/red wires that feed the fog lights.
So your assumption is correct.
These rear fogs only work when the dipped beams are on. When main beams
are on, or only side lights are on, the fogs go out. Edit: the warning lamp
in the cluster mimics the fog light operation.
(However, I see you're in NZ,
and lighting regs may be different to UK, hence possible operational variations)
The grey/red cables run down each side of the car to connectors above the bins/speaker
area. Check there for continuity.
Hope this helps.
Nigel