The Onstar unit is in the trunk, on the driver's side. Pull the trunk lining carpet away from the side of the trunk, and you will see it. The back-up battery is mounted on the side of the unit with a special bracket. Follow the instructions on the tech sheet listed above to avoid the check-engine light becoming lit. The battery is the "Ultralife" brand and has a GM part number of 88861186. Cost is 54.04 from www.gmpartsdirect.com
. Follow the instructions in the GM tech sheet listed above and do it yourself. Save the labor cost. I hope this helped!!