for 72 and 73 the system works like this:
-key in buzzer... is the horn relay itself ( same for 71 ). Gets a door ground just from driver side door switch ( black wire ). When opens the door sends ground to the column switch sensor switch. If key is IN the circuit is closed and makes buzz the buzzer. Buzzer gets constant positive
-Lights on buzzer... Gets door ground from same source than previous system. Gets the lights on buzzer. At the time, the positive source is sourced from light switch PARKING position. Once both switches are closed feeded both poles, the lights on switch will buzz. This buzzer is located just right at side of ashtray receptacle attached to dash frame.
-Seatbelt buzzer and light... Gets a relay ( pictured ) which gets two poles ( ground and positive, however the relay is chassis grounded too ) some like the lights on buzzer, just that the positive comes from RUN circuit ( blue wire ) and ground comes from NSS ( brown traced ) spliced from starter relay. Once Run circuit is closed and Ground signal from NSS is cut, relay sends GROUND retractors and seats sensors.
When retractor and seats sensors gets this ground meeting all the requirements, circuit keeps closed this ground to make buzz/light the seatbelt warning system. Positive is constant from RUN circuit on these.
This buzzer is a plastic one, attached to far right side on dash frame.
View attachment 268210
as you can see every buzzer is independient
in 74 everything of this was TOTALLY redesigned.
- - - Updated - - -
for more ( although showing more functions being to Imperials ):
http://www.imperialclub.com/Repair/Lit/Master/294/index.htm