Won't crank

Hey guys /gals working on my uncle car and having a issue with cranking car is a 73 duster 360...yes 360 with a auto trans car won't crank I can get it to if I jump post on the starter relay but not buy the key it's self thought maybe neural safety switch but I do have reverse lights and if I can rember right they use the same switch for both I have try to start In neural as well still nothing and the items that don't have juice while cranking go out such as the radio and stuff so that part is working any ideas where I should start looking thanks for the help also a reminder I do suffer from a visual disorder so please be kind and understanding I'm doing the best I can