Diagrams from MyMopar:
http://www.mymopar.com/downloads/1964/64DartA.jpg
http://www.mymopar.com/downloads/1964/64DartB.jpg
What might make sense is this is the fuel gauge sender wire, and the original connector for the cluster is AFU. The fuel gauge I believe has a stud / nut for the sender wire, which normally jumpers over to the cluster connector.
You could find this out right quick by
1....checking this unknown for power
2...If not, then check resistance
3....Then access the kick panel connector for the rear harness and separate it, and re-check resistance. If it goes open, you have found it
Also check the cluster connector and see if the light blue at "J" going to the sender is out of there