I had the same problem with my 68 coupe about 3 years ago. My original harness had been cut/spliced in over 30 different locations and battery was draining every 3 days. Was willing to fork out $550 for a new repro harness but at the time nobody sold one for a 68. I located a salvage yard (Georgia I believe) called Mustang Works and ordered a used original from them. Granted the wires were still over 30 years old but it was flawless with not one splice. Fixed all my problems. They charged me $250. If you're interested, their web site is
www.mustangworks.com/. Keep in mind the underdash harness does not include the instrument panel harness. They might sell that separate if you need it.