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.