Check the connections to the starter relay, they should be clean, what
is the voltage to starter? Also check resistance to ground from the
battery just for good measure, should be none if I remember correctly.
If the ground is good you can try connecting a jumper cable to the
positive battery terminal and touching the other end to the starter
terminal. If you get a crank check the components between the battery
and the starter, i.e., starter relay.
I jump started an old dodge like this before, YMMV. I think it had a
bad ballast resistor, I forget.