I've heard about this "feature". What the conventional wisdom maintained was the output side of the fuse (fuse tap clip and lead) used for the radio was only put in the fuse block when the radio was used. The wiring diagram for 64 Mustangs (Assembly Manual) does not have radio wiring included but the extension lead is shown on the radio installation page as p/n C4ZB-18A919-A. For 65, it's similar, there's a triple tap add-on lead, C5ZB-14A318-B, identified as an "accessory feed", used with an extension lead, C5ZB-18A919-A, from the triple tap to the radio. For 66, the triple tap lead is part of the fuse block, the extension is the same.
Used is about your only source if you're lucky enough to find one.
Jim