Does anybody know where these plug into?

Since the seat belt interlock has been brought up, this may be of interest to some of you. This is a quote directly from a TSB dated November 1,1974.
On October 31,1974, Chrysler Corporation began building vehicles without the interlock function at some plants; the other plants will follow in a day or so. Vehicles built without the interlock can be identified by a green control unit case with yellow tape applied.