On my 74 that is for the seat belt warning system. I took mine out it has a harness that runs under the carpet to the sensors under the seat. I have the harness if you want it and the sensors also I believe.
In a 74, that plug is for the seat belt warning system if it is on the passenger side. I just removed all that from the harness on my 74 Duster prior to reinstallation.
I had not noticed this was on the pass side. Above is correct. Look under the hood for the reset button. You must bypass that by connecting the two yellow wires going to it
