Ignition help needed. 1973 Duster

If it is auto trans, the relay gets it's ground connection from the neutral/park switch on the trans. Check out the shifter adjustment/wiring. The switch could also be faulty. So could the relay.
Get somebody to touch the relay as you turn the ign switch to the 'crank' position. If they feel a click, the relay is working, but there is a problem with the relay contacts &/or wiring if the engine fails to crank.
If there is no click, relay could be faulty, ground missing [ as described above ] or 12V to relay is missing.