It might be, but my recollection is the seatbelt reminder light and/or chime also existed seperately from the interlock.
Electric defrost grid is a resistance heating material on the glass. Different than the hot air blower units (also an option) and not related to air conditioning. How rare is electric rear defrost?
The MTSC for the Seatbelt interlock can be found here Browse MTSC by Model Year – 1974 – MyMopar
Page 1 has the explanations and note that it says the circuits are entirely different than the warning buzzer system.
Page 8 explains the overide switch