Not sure about the early cars, but the later cars indeed have two buzzers. They are little box-shaped plastic items under the dash. The key buzzer should have two terminals, and the seatbelt buzzer should have 4 terminals.
Sometimes the key buzzer contact gets worn or removed inside the column. After you ensure you have both buzzers installed (many times they're removed b/c people find them annoying), open the driver's door and push the key in the ignition. When I got my 80, it had a bad contact so it would only buzz if I actually pushed the key against the ignition (lol). If that doesn't work, you can pull the steering wheel and lockplate behind the wheel to see if the buzzer contact is installed.
I'm not sure about the seatbelt buzzer wiring. I believe it ties into the seatbelt dash indicator light on the speedometer, but someone with a wiring schematic can probably better elaborate on that.
