First, the part numbers.
The "clip" in the first post above is 372302-S100.
The "clip" in the second post (reply #1) is 372047-S100.
Neither are used for fog lamp wiring on either 65 or 66 fog lamps.
What is called out are two straps, 376914-S, which is the primary retainer for the 64-66 headlight harness (basic Ford number 14290) and the RH side fog light harness shares the left strap.
Now to the exceptions: Clip 372047-S100 was used in 1966 as additional support for harness 14290. It is not used with the fog lamp harness. This "clip" seems to be a San Jose only use.
Clip 372302-S100 is not used with fog lamps, however, it "may" have slipped in.
Ref: 1965 GT Fastback, 5R09219xxx (5-11-65 scheduled); 1966 GT Fastback, 6R09K121xxx (10-18-65 scheduled).
I recommend you get a set of 1965 Mustang Assembly Manuals for your car. Include the 64 Chassis and 64 Weld and Sealant manuals.
Jim