Brian : 65's aren't my specialty, nut I can provide the following ; unfortunately, nether my 1965 Ford Facts book (dated 8/64) nor my Salesman's Price Book (dated 9/25/64) even note the availability of the GT Option (it apparently came out later). However, the 1965 Mustang Sales Brochure (dated 4/65) does list it (copy attached). As you can see, Back-op Lights are not listed as being included with the GT Option (at least at the time of this brochure).
Remembering that Sales Brochures just give an idea of what was intended at the time of their printing, and are not considered "Gospel", I would look for further documentation (a later Price Book, TSB, etc.) to confirm this.
Before Posting this, I remembered one other of my resources to check - my Ford Shop Tips. Sure enough, the last page of the May-June 1965 issue (copy attached)n introduces the Mustang GT Option.
Bob
P.S. Just to be clear, as rocket289k noted, Back-up Lights were available for all Mustangs in 1965, but not part of the GT Option.