So, you want to tap into the lights wiring, pick up the 12v when the lights are on, and trigger your own relay?
You're making this waaaaaaaaay too hard...you have a GT, right: why not use the foglight feature on the headlight switch (pulls out when the lights are on)....and the foglight wires in the harness behind the grill??? Or do you want the fogs to come on all the time automatically, regardless (which might not even be legal). :nono:
In stock form on a GT, you have the relay in the BEC. The stock light switch tells the SJB when to turn on the fogs, and the SJB energizes the foglight relay in the BEC. Hell they might even plug direct into your lights as-is. Worst-case you splice into the foglamp wires that are there.
If you insist on forcing the foglamps on when lights are on, you can wire up the relay inside the cabin. But if you're having this much problems with the wiring, that is not something you should tackle.
Now, if you have a v6 and are posting in the GT section, this is quite different, as only part of the wiring and circuitry for foglamps exist, and you have to hookup the rest. I had to do that.
Also, relays have 4 or 5 pins, typically numbers 85 and 86 for the energizer coil circuit, and 30 and 87 (and 87a) for the contacts. Your power from battery-to-whetever-is-to-get-power goes through the 30/87 contacts. Power and ground to control the relay go through 85 and 86.
You might think that the typical relay wiring in these Fords is that one of the coil pins (85 or 86) is grounded and power is switched on to the other when the relay is to be energized. That's simply not the case...just the opposite occurs in the factory wiring in these cars: power is on to one side of the relay coil (85 or 86), and then some control (a switch or the SJB) grounds the other side to turn the relay on. This is how all of the circuits work in these Mustangs.
Nonetheless, there is a +12 signal that comes off the light switch when the lights are on (brown wire, pin #9 I think; I use it to tell my gauges when the lights are on). This cant feed foglights, but can be used to trigger a relay.
Note that if you have an automatic, there's a large and easy-to-get-to hole in the firewall, between the brake booster and fender, with metal tape; you can poke a wire through there very easily. Inside there's some foam left of the brake pedal, with the section cut-out yet still in place. Simply remove that section and it's easy to run wires in/out of the cabin. This 'hole' is used for the clutch, so if you have a manual, this doesn't apply.
But the grommets on each side of the cabin, behind the splash shields, each have a sealed 'nipple' that can be cut or poked through and a wire or wires run through it, very easily.
Just giving you some options and additional info....there's many ways to skin this cat :bigthumbsup