I always love a mystery.
First of all, Cruiser68
, did you buy the grille, lamps and harness all from eBay? Whatever, they didn't sell you a matched set. Looking at various sources the 68 fog lamps should have a single blue(?) connector instead of the 2 separate wires on your lamps. Those are lamps for an earlier car. OK, I see MIL1ION
has just posted what looks like a more correct lamp wiring. VCM sells the 68 fog lamp pigtail with blue connector for $10. However, the Scott Drake photos, as well as the one from MIL1ION
just above, seem to indicate that the correct '68 lamp wiring won't mate with your blue harness connector. Did they sell you the wrong harness as well?
Second, I agree that the '68 fog wiring schematic is hard to come by. The '66 seems to hide also until I found it right here on the site. Look up at the top of Classic Tech, take the thread and link marked for Wiring and Vacuum
and look under '66 for Exterior Lighting. Not the schematic though; the pictorial. In the upper left corner of the External Lighting pictorial there is a schematic of how the fog lights are wired.
, I'm not sure you have that wiring exactly correct. I doubt there was any ground wired into the fog switch and also doubt there are any grounds in the fuse block you can connect to. Ground on these cars is carried through the steel of the chassis. That wire confused with a ground is almost surely described by a much younger Veronica in this thread from 5 years ago: 66 foglight wiring diagram, NEEDED! - Ford Mustang Forums
(V: May I suggest a medal for longevity of technical excellence?)
All of the early fog light switch wiring pigtails look much the same and I doubt they are wired any differently. The 3 wires on the fog switch are 1) power to the switch, 2) power to the fog lamps (gray) and 3) power to the tail lights which should come on with the fog lights. Another hint is that the wire colors from the '66 schematic are still the ones being used on the '68.
The first pictures above don't show what happens to the black wire on the engine side of the blue connector. I think it is a ground lug that is grounded on the radiator support metal like is shown in the '66 schematic. You have to somehow modify your lamp wiring to match the connector, get lamps with wiring that matches the connector, or take the fog lamp ground wire directly to ground without benefit of the connector.
If the 68 is somehow vastly different, I apologize in advance. But I think you will find that the above links will tell all you need to know.