How do you replace and where can I buy one
The additional battery is located under the instrument cluster. You have to remove the cover on the side of the dashboard and remove the cover just beside the brake pedal. Then the bracket inside must be loosened and moved to the side with the electrical connectors on it. Remove the battery wires, remove the screw that holds the battery in place, remove battery and bracket downwards from other bracket.
As far as sourcing, I would buy from MB. They're not too expensive. Otherwise they show up on eBay now and again.