I had something similar, but it was orange/red and it was mounted under the hood of my '65, it was some sort of NOX device for the California smog test, it had vaccuum lines and a wire off to a power or something, that was 20+ years ago.
i've seen similar boxes on other fords but never in a 70 stang. possibly for the transistorized ignition used on some california models. could be a headlight warning buzzer, key in ignition reminder buzzer or seat belt reminder buzzer. i think that's what the seatbelt reminder buzzer in my old 73 galaxie 500 looked like IIRC. it's been a long time since i had that car though. i do remember a similar box on it though. only thing i can remember it having was the seatbelt reminder.
are there any numbers on it anywhere? if so post them and maybe someone can cross them and see what it fits.