Security light flashing

Possible damaged key. Read up on GM security systems. http://www.lockmasters.com/site/pdfs/TT4002%20Vats%20Overview%20Article.pdf

As a remote start installer this one's an easy fix. First off, do you have a remote start in it? Some of the bypasses we were taught was to just find the resistance of the passlock (yellow wire) after cutting it, then adding a resistor with the proper value from the black passlock wire to the car side of the cut yellow wire. Some shops may have used cheap resistors that fail after a few years. There's a sequence you can do to program the truck to whatever resistor you put in that spot but do not go over a 7k ohm resistor, or was it 12k? 10 minutes key in the on position, then within 10 seconds turn the key off, then start it. You must time it right.

There are ways to bypass this this key issue now. Read through this page for info, solutions as well. http://www.diy-gm-security.com/