When security light flashes the PCM is receiving a no-go signal or no signal at all from the anti-theft system, the computer won't enable the ignition, fuel system or starter and the engine won't start when you turn the key.
Most common problems related the the Passlock family involve the Passlock sensor. The sensor is part of the ignition lock cylinder on Passlock I and Passlock II systems. The antenna is the Passlock sensor on Passlock III systems. If these sensors fail, the proper repair would be to replace the failed part and perform a theft system relearn procedure.
By far, the most common problem with the VATS system is broken wires in the steering column. These wires connect the ignition lock cylinder to the under-dash wire harness. The proper repair is to replace the ignition lock cylinder, which comes with wires already attached that snake down the steering column. The new lock cylinder will come with a key blank that will only rotate the new lock cylinder" it does not have a resistor in it that will allow the engine to start and run. A new key must be made from the blank because it has the proper resistor value. Normally, the resistor is read from the old key using a tool called an "interrogator." The interrogator is available in the parts department of most GM dealers. You must use a scan tool to perform the BCM theft deterrent relearn procedure whenever you replace the ignition switch lock cylinder with a special scan tool.
Do your homework by checking corroded connectors just the tamper sensor or security sensor is bad .
(I'm sending this diagram to your email to help you see what is involve)