Starter Engaging Issue

Hello, I am new to the forum, and was looking for Relay diagrams for a starter cutoff. Such a wealth of information, thanks to everyone for making it happen. So here is the issue. My friend has a 2004 Chevy Express cargo van. He needs the van for his profession (special effects in the movie industry). His van all of a sudden started making this horrible clanking noise, but it was intermittent. He took it to a mechanic the first day it started making the noise, and they said the starter was engaging at its own will while the van was already running. They recommended a well known electronic place, so we had it towed over to the shop. Numerous attempts to find the issue, rebuilt starter, bypassed the aftermarket alarm that was installed. It would run fine, and the noise would come back. As soon as the temp dropped it has been immediate everytime we start the van. I replaced the starter relay (new delco) and cleaned the contacts in the fuse box. No change. I even took the van to the installer of the alarm, and the confirmed that the starter kill had been bypassed, and they even unplugged the alarm all together, and no change. Just clanging and grinding, but only in Park and neutral. It appears the neutral safety switch is doing its job. I could keep diagnosing this issue, but I decided a simple momentary switch in the starter signal wire may solve the problem. Hence why i was looking at relay kills. But the one I was looking at in the FAQ on this forum (Starter Kill, Passive w/ switch) requires two relays, and seems a bit complicated. It also states the power to the starter, continuity is broken on the starter feed wire. I simply want to put a switch to stop any unwanted signal going to the starter. Any recommendations? I am about to torch this damn van, but I know it is a simple wiring (ha) issue, and if I were better at diagnosing, I could fix it. But sometime a Jerry rig gets it done.

In order for the starter motor to engage, the neutral safety switch must be in either park or neutral and the PCM must enable the starter relay. For both of those to act up together when cold is unlikely. I would suggest that you may have a corrosion concern in the underhood fuse box. The older GM full size vans have been know to have water damage to the underhood fuse box cause many weird electrical issues. There should be a plastic piece clipped to the fender where the hood closes on the driver's side. It is designed to direct rain forward towards the grill like a rain gutter. Remove the underhood fuse box and check for corrosion or a short circuit. The starter solenoid wire is a purple 10 gauge wire coming from the fuse box to the starter solenoid.sparky