Starter stays cranking!!

[HD2500]

I'm trying to get to the bottom of an issue with my Dad's 03 GMC 1500HD.
Problem is, usually the first start or after it's been sitting, you turn the key to crank, but the starter remains engaged without actually firing the motor. It will just keep cranking unless you turn the key off... Typically the following attempt to start it, the engine will crank and fire normally. I've done all the research on this problem that I can, just not coming up with any definite answers.

Here's what I've replaced:
-Starter (twice) & played with different shims
-Ignition Switch
-Starter & Ignition Relays
-Ignition Key Tumbler

Nothing has affected it at all. My best guess is that when cold, something is making a connection that keeps sending power to the starter solenoid. Then, when warm (after a few consecutive starts) that connection is lost, or no longer bridged. The next place I'm going to look is the Neutral/Safety Switch.

Any ideas?

 

[Bonecollector]

Has the truck ever had a remote start in it?

Have you checked all the connectors for moisture? Moisture provides an excellent path for electricity to jump around


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[HD2500]

Has the truck ever had a remote start in it?

Have you checked all the connectors for moisture? Moisture provides an excellent path for electricity to jump around


Sent from my iPhone using Tapatalk

nope, never had a remote start. and I have only checked the connections around the parts I've replaced. Where else should I look?

 

[Bonecollector]

If anywhere it's probably where connections are exposed to the elements, try dielectric grease in the connections that relate to the starter


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[HD2500]

I'm going to investigate the Neutral Safety switch and it's connections this afternoon when he gets home. Plus I'll look for any other exposed connections as well.
Found this diagram and I'm betting the problem lies within.