1975 Plymouth duster brake relay question?
Here's a few pictures of the one that was on my '74 Duster. And no, I didn't put the wire nuts there, and yes, they were dealt with before the car hit the road. These pictures were taken right after I bought it, when I had the original /6 out, before I swapped the small block in and fixed some of the questionable things some of the PO's had done. It is in fact a reset button that resets the relay if you trip the interlock mechanism.
The seat belt interlocks were a pain in the ***. Mine were disconnected when I bough the car, but even with the seat belt switches disconnected the interlock can still kill the ignition if any of the original interlock electronics fail. The reset relay shown here later went bad, and when it did it disabled the ignition. I bypassed it later, but even then there are other components in the system that can come back and haunt you.
With as chopped up as your harness looks, I would look for a complete '73 engine harness.