[HoosierDaddy]

What John said. Sometimes if you just tighten the nut that holds the large batt cable to the start motor that is enough. Whether that works or not it is good practice to remove that cable, wire brush the mating surfaces , apply some grease and reassemble.
You could rule out Carls idea by checking for voltage at the start solenoid primary (small) wire while an assistant turns the key to start position.
Hmmm I should have read your addendum!!