Is there a factory alarm?

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Not really a bunch of BS. GM field service engineers, along with service managers and technicians face this problem everyday. Aftermarket alarms are noted to drain batteries, cause no-start conditions, due to improper wiring and/or components.
If an aftermarket alarm is installed correctly with good reliable products, it should be no problem. The problem lies with finding an aftermarket installer who really knows what they are attaching to in the electrical system. That is provided the customer purchases a reliable system, not one of those hunky, cheap systems that look appealing.
You also have to take into consideration that if you install an aftermarket alarm, if it causes a problem, GM will not pay for the work, you will have to pay and chase the installer. If a dealer installs the alarm, at least he can't point fingers and will have to cover the repair.
Also, most insurance companies only offer a discount for a passive system; one that sets itself after a certain period of time. An active system is up to you to set. That could be one reason why the insurance company dropped the discount for the system.