73 duster wont crank

How are you jumpering at the starter relay?

The safest way to do it is to disconnect the spade terminal (yellow wire, Packard 57 1-pole) and apply 12 V to it. The closest place to get +12V is the big stud on the starter relay, which is direct from battery on all I have worked on. This method actuates the relay the same way the starter switch does, i.e. how criminals "hot wire" under the dash. It is safest because the circuit still involves the "neutral safety switch" (NSS) in the transmission. If this works, it indicates your key switch is bad or not getting power. If it doesn't work, either the tranny is not in P or N, the NSS may be bad, or the relay may be bad. You can ground the NSS terminal to bypass it, but insure you are in P first.

The other way to jump is to connect from +12 V (big stud) to the starter coil wire (small screw ~8 awg brown wire I recall). Make sure the transmission is in neutral or the car may jump forward and pin you to the wall. If that doesn't work, your starter may be bad. If you hear the starter coil click, but nothing, its contacts may be pitted, or your battery may just be low.