Could you verify the year. A build date of September 2, 1965 would be a 66 model Mustang. My notes say that a 66 main harness has one tag located at the spot where the gauge, coil and heater wires go thru the fire wall. The tag is yellow with blue letters for the part number. I didn't note the source. Other notes show a p/n of C6ZB-14401-B, -C, -F and a -G. I assume there were running changes, for the emergency flasher circuitry possibly.
Jim