I'm not real sure about this... but I think the 75 had a connection on the underside of the seat, check to make sure it is making good contact.. best way is to unplug it a few times then plug it back in, if all else fails you can leave it unplugged and that should stop the light from coming on.. This is if I'm right about the '75.. a quick check under the seat will tell ya.. good luck.
Could be that your timmer/buzzer assembly is shot. The assembly is plugged into the fuse panel and has couple of jumper wires also plugged into the fuse panel. This assembly operates the key buzzer, lights-on buzzer and times the seatbelt reminder light.