Here'* some info I dug up on DERMs (dated 2005)
This device is often refered to (incorrectly at that) as the "black box". The DERM module has three functions:
1) Determines how and when to deploy airbag(*) and at what velocity. (On newer vehicles beyond 2004) It also "Stores" a small amount of current (up to 15 minutes, standby capacity) in the event of battery failure in a severe frontal collision, still allowing deployment of airbags and logging of data.
2) Records "near collision" data, and retains this for 250 Key on, key off cycles. (It can tattle on you being an agressive driver if the data is pulled from it within this time period!)
3) Records permanently the last 5 seconds of information up to a collision and airbag deployment. It records at it'* most basic level, Throttle position (percentage of Wide open throttle), Wether the brakes were applied or not at the time of colision, vehicle speed, and engine RPM. If the vehicle was rebuilt from a crash, this module would have to be replaced as it remains in a permanent EPROM flashed state from the last accident it recorded.
In 1998 and later Blazers, this module is located under the center console on the floor. Data is fuzzy/nonexistant on earlier models that I am looking for at the moment. Following GM'* logic though, I would say to look for it under the passenger seat, or all along, under the knee bolster. It will be easy to spot because it will have a yellow conector on it and a yellow and silver sticker on top of a shiny aluminum box.
Try your luck at a Junkyard for locating one from a vehicle that was not wrecked. Most junk places I have gone to for various bits and parts over the years just shrug when they see a small part and say "Gimmie $10/$25 for that." It'* useless to get one from a wrecked vehicle because it will most likely been "locked" with last known collision data stored and cannot be reset. You'll have to get a virgin one, or one from a car/truck that was wrecked without airbag deployment.
Shopping around on E-bay and parts house specializing in airbag modules reveals a steep discount compared to the dealership quotes $600+ dollars, but - this IS a component you don't want to be caught out on with the wrong part, incorrect installation, etc. What kind of price-tag can you put on your safety and get this done right?
Also, beware of E-bay and Internet deals on these modules at the moment (2005). Thousands of cars and trucks have been drowned in floods this year alone (2005).
There are businesses springing up all around that claim to clean and "reflash" these modules to "like new" condition after they have been flooded and rendered destroyed in the eyes of GM. I would rather pick one up off the shelf at the dealership, sealed, brand-new in the box, IMO.