I haven't been on here for a while but when I happened to get back on here I see all kinds of posts of misinformation about how to disable the Passlock systems in most '98-'06 (maybe more) GM vehicles. DO NOT use the toggle switch trick. That's just a way to diable the system into thinking it's OK to run in the "fail, enable mode". Most of the time if you have to unhook your cars battery or if the car goes dead, you will never start the car again unless towed to a dealer.
Forget about buying pricey alarm bypasses or even the guy on the net that has the "solution" module to bypass this. Parts to do this on your own shouldn't cost more than $5.
Passlock in these GM vehicles consist of 3 wires. Usually these are a yellow, white, and black (or orange/black on trucks)
Now you can "read" the resistance if you want and try to match it. Problem with that is that value may be just on the fringe of getting the passlock system to trip and cause a no start condition. What I like to do is to pick a random value, get the system to lock out, then do the GM relearn sequence.
Get a resistor (somewhere around 500-2500ohms seems to work best) Cut the yellow wire. Connect one end of the resistor to the end of the yellow wire going AWAY from the ignition switch. Connect the other end of resistor to the black ground reference wire. Then, take a jumper wire from the ground reference wire and the other end to chassis ground. Try to start the car. It shouldn't and the theft light should flash. Leave ignition on for 10 minutes or until that light stops flashing. Turn car off and count to 3. Try to start. Normally it will but if not, do the 10 min. trick again. It can take up to 3 attempts. If after 3 attempts it doesn't work, change resistance value and do it again.
Remember you with trucks your reference wire is orange/black. I own a car bizz and I researched this over the last 5-6 years now. I have done 100's of GM vehicles like this and had about 99% success rate. Now, your vehicle could have a BCM failure in which this will not work. Normally though if you have a BCM problem it will be associated with some other qwerky things as well.
THIS INFORMATION IS BEING PROVIDED AT FREE WILL AND I DO NOT TAKE ANY RESPONSIBILTY OF ANY DAMAGES THAT MAY RESULT IN THIS MODIFICATION.