I just bought a 1997 GMC Jimmy with 92000 miles. Truck appears as though it has been well maintained. The one thing that it does that bothers me is that when you start the truck, the doors AUTOMATICALLY lock. So if you go out to start your car to warm it up without thinking and shut the door....your in trouble. Anything you can do to stop this from happening short of pulling the door lock fuse

Does it have an after market alarm system at all?

Nope...stock SLS

Check your owners manual, there is a section on how to reprogram the door locks. There are several options.