The highs break the switched ground for the fogs, so the fogs would be unswitchable if you simply grounded them.
You need to prevent the high beam circuit from breaking the ground for the fogs. It'* very clear once you see the circuit in the FSM, and it'* not as simple as you might think. The Lamp Control Module has control of these circuits, not the PCM (at least in my 93). Honestly, the mod I did was the simplest way I could find of getting it done.
Page 2 has the details. This should work on 92-94, possibly 95 and newer. You'll have to locate the parts, but a FSM would help alot.
Again, simply grounding the highs won't work on my car, nor will grounding the fogs.