With the help of a lot of forum friends, I've finally wired the fogs on my 67. I've listed the process below, in case it's useful to others. Here's what I did (using Mustangs Unlimited's fog light kit and their 10amp circuit breaker which did not come with the kit):
1. Hooked up the fog lights to new black ground wires and grounded them to the grill (couldn't find the factory grounds that may have been taped inside the existing harness).
2. Hooked up the fog lights to the new gray power wire and ran it along the engine's maine wire harness and through the firewall.
3. Then hooked that up to the underdash Feed Harness (came with the kit) via a bullet connector/gray wire. This gray wire then goes to the Feed Harness' plug, which plugs into the fog light switch (the Feed Harness plug has three wires coming out of it, the one just mentioned and the next two below).
4. Also coming out of the Feed Harness' plug is a black male bullet connector with black wire which I connected to the black female bullet connector over by he headlight switch (to engage the tail lights). I removed the heater controls panel and found the factor black female bullet connector right away.
5. Also coming out of the Feed Harness' plug is ONE Blue Black wire that ends in a closed loop connector. I attached that to the post on the Circuit Breaker marked AUX (or something to that effect that suggested "auxiliary"). Taped (but not connected) to this Blue Black wire is another, very short Blue Black wire. On one end is another closed loop connector, which I connected to the Circuit Breaker post marked BAT. On the other end of this short wire is a male bullet connector, which I plugged into the car's factory harness Blue Black female bullet connector (I'm told that this factory Blue Black is spliced into the car's Yellow Black wire that runs to the battery).
6. I mounted the fog switch directly under the ignition. But what I didn't initially realize is that the fog switch isn't really visible when mounted. The switch points down towards the floor of the car and you have to basically look under the console to see it (I'd assumed it was supposed to be mounted somewhere on the dashboard/control panel). There were holes pre-drilled to mount it. I then mounted the circuit breaker right next to the switch, but up on the underside of the edge of the dash (there were two holes that lined up perfectly with those on the circuit breaker). I'm told that the circuit breaker should be mounted up on the windshield wiper motor bracket, but that would have been tough to get to.
So that's how I did it. It's working well, so I assume I've done everything correctly.
Below are two links to other forums with two very useful diagrams. The first is a hand drawing from Mil1ion, the second is a fog light wiring schematic for a 66 Mustang, which is more or less similar to the 67 process (since it seems that a 67 wiring diagram doesn't exist):
67 fog lamp relay?
Help! GT FOG LIGHT INSTALL