Maybe the starter motor its self is buggered, I have to pull the starter out of our 6 cyclinder VC it sometimes is just dead when you turn the key, it is the brushes in the starter its self, if I give the starter a hit with something heavy it fires up, but a few days later or when hot dead again. Get a test light and see if power is going to the solenoid when someone turns the key, if there is no power there when someone is turning the key it could be the ignition switch or the neutral safety switch if it is an auto.
From memory on our 6 cylinder there is one wire going to the solenoid a push on type, the large power cable from the battery which bolts on to a post, and there is another wire on the same post under the thick cable, ie 2 wires on the thick post.