I own a 72 BGT (non-od) and I have two switches on my transmission. One
switch is definitely for the reverse lamps. The second(Light Green w/White)
is wired into the drivers and passengers seatbelt buckle switch, then to a
switch in the passenger seat, then to a line diode and a light on my console
that says "Fasten Seatbelts" and finally to the ignition key warning buzzer.
When the circuit was working, I recall that the "Fasten Seatbelt" light was
illuminated when the car was in gear and you were not wearing the seatbelt.
I can't remember if the buzzer also sounded. It appears that this switch
may have been used for various applications over the years.
BTW, I am new to the list and in addition to the GT, I am the original owner
of a 77 B which is virtually factory original(like any 2 mgs ever came out
of the factory the same!!!) If anyone is restoring a 77 and needs some
"originality" questions answered, let me know.