Hey the best advice I can give you is that if you really want to make the job as professional as possible you can buy the harness all ready to install, but nee a switch and you are in business. I just spent a whole month researching it and finally I did it. So this is what I did, my fogs and my harness along with the switch look straight out of the factory like GTs.
Regular fog switch, which I bought from eBay for $12
American muscle roxio fog lights got them during memorial day for cheap.
I ordered a custom professionally made harness from this seller.
1999 00 01 2 3 4 Mustang V6 GT Fog Light Wiring Harness | eBay
this is the harness I got. and it was a two hour job easy and painless.
if you are going for original look don't hold back. I didn't and could be much happier .
2004 40TH Anniversary Oxford White Mustang V6 3.9lt
MODED.
H&R Super Sports Springs
Koni Adjustable Shocks and Struts
Steeda Camber Cater Plates
Steeda X2 Ball Joint and Bump steer kit
BBK Cold Air Intake
JBL CERAMIC HEADERS
MAGNAFLOW Catback Dual Exhaust 4" Tips
Mach 1 13" Front Rotors and Calipers
BLACK ROXIO HEAD LIGHTS
Recent Tune Up
GK Iridium IX Performance Sparks Plugs
10mm Performance Distributors Live wire
MSD Performance Coil Pack
and all Royal Purple oil 5w-20
and new
ROXIO FOG LIGHTS with custom made harness, the cool part about the harness is that the fogs stay on when you have the high brights on.
Hope this info about the fog light harness can help you.
I will post pictures on the fogs and other upgrades I have done.
Thanks.