Seatbelt light began on 71 being time delay relay controled.

Then 72 and 73 began to be controled by seats and seatbelt retractor sensors, just as a warning buzz and light system

74 began to be interlocked, also with sensors ( this time seatbelt sensors on female buckles ), but just on USA models. Canadian for example even still working and wired the same as USA models, still allowed to crank the engine using a diff control box unit.