Greetings Chris!
I can't say with great certainty, but I suspect that the extra wire may be
for the optional backup lights. I know this was an option in '64 and assume
that they were in '63 as well. The literature that came with my '64 made
mention that the backup lights were available as a factory installed or
dealer installed option which leads me to believe that the wire must have
been placed in all trailers even if the option was specified at the time of
purchase. I have considered checkin in my Overlander as I really like the
backup lights on my Minuet - - not so much that they help me - - rather
their ability to warn approaching drivers when I back my trailer into the
side driveway at my house (I live on the main approach route for our
hospital's emergency room so there is quite often unexpected traffic on the
street).
Good luck with solving the riddle of the extra wire!
Kevin
Kevin D. Allen
WBCCI/VAC #6359
1964 Overlander International/1999 GMC K2500 Suburban
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado (Towcar-In-Training)