1975 Plymouth duster brake relay question?

It might be that you have an off model and it may be the 1974 seat belt interlock. To find out, pull the connector and see if the the key will operate the starter. If that happens, you need to bypass it. Should have two yellowish wires, which you need to jumper together. They are the start wire going from the key to the start relay. That box, when tripped (if seat belt) opens that circuit

This was a Federally mandated "one year only" deal in 74, and EVERYBODY hated it.