It could be the neutral safety switch (I'm assuming this is an automatic transmission). You could just by-pass the switch with a jumper wire and see if that works.
If not, then it sounds like your problem is in the ignition switch itself. You might poke around and look at various fuses and fusible links as well. I presume from your last post that you applied current to the starter relay so we know that works.
Probably it won't stay running because you don't have power from the ignition switch to the glow plug circuit.
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