1998 GMC C3500 Security light

Your vehical has a security system called Passlock. It relies on a resistor value to activate the ignition system. The resistor is probably going bad, but you must take it to a dealer to fix this. However, you can temporarily bypass this and start the engine by doing the following---
1--turn the key on but do not crank the engine
2--the security light is on or blinking
3--wait until the light goes out or stops blinking, this takes about 10 minutes.
4--now, turn the key off
5--within 3 seconds turn the key and start the engine.
This will bypass the passlock system for a while, but to fix the problem you must see a dealer.