This is wonderful! Thanks for taking the care and efforts to do this. The SPST switch... What is that?

It is a single normally open switch Single Pull Single Throw SPST

It is hooked up as shown in the diagram-


The volvo front fog switch? I have a second spare switch if that matters. I think I understand the relay... Mine has power wires (black) going to the lights, a ground wire (blue) a yellow, and greenish wire that go to the switch along with a ground wire (blue for the switch... and the positive (red) wire to the battery.

This is crazy! The relay needs identification for the coil contacts
and the switching points. Hook it up as shown in the diagram
typically C=common NO= Normally open N= Normally closed
Look carefully at the relay for the pinout.
Relays usually have the pinout on them.
Use the points specified in the diagram.

The switched accessory is any switched power source? I'm guessing that's where my green wire goes.

Yes- 12VDC appears at the radio or acc outlet switched by the ignition switch.
These are fused and safe to use.

More: Volvo fuses fog lights at 15 Amps so that's minimum 14 gauge wire to the lights. more than a single light, hook up in parallel, not series.
The 850 Fog lights work with the low beams only
(High beams are no good in fog)

Leave your phone nr if you're stumped.
Do you have a name? :-)

Bill