Sometimes the BCM can have issues, but a lot of the time it is in the ignition cylinder like suggested, there is a bypass made that will allow the vehicle to start no matter what part of VATS is bad.
Usually the security light will stay on when it is a VATS issue, the fact that it comes on, then goes off in a few seconds makes me think it may not be a VATS related issue.

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When all is well, the security light is supposed to come on for a few seconds, then go out. If it continuously blinks, it means it reads no resistance (broken wire usually) or if it stays steady, it reads the wrong resistance.

Hmm. I'm finally able to do some more testing on it tomorrow, but I'm starting to lean towards a grounding/solenoid issue now.

Thanks so much everyone for the help so far.