While prorac1 did a very nice installation job with the AM/FM radio, based on the font on the faceplate of the AM/FM they used it looks more like a '66-'67 "B" body AM/FM than a correct '73 scamp one even if removed from a scamp. This would also explain the difficulty in exchanging buttons another member mentioned as I've not had a problem doing that between AM's and AM/FM's as long as the years were the same or +/- 1-2 yrs. approx. Agree with other members that the '73?- '76 style knob type AM/FM is the best solution for your '66 cuda as it has the same white numerals on a black background as your '66 cuda (and many others) and is a direct bolt in, including knobs and wiring, other than a minor mod on rear support brace if used. Never seen the third wire your referring to, and I've looked at many of these radios over the years from a large variety of years and applications.