When they installed the battery they may have screwed up. Crossfires/Mercedes are very temperamental when it comes to voltage spikes.
When removing the battery, the negative cable should always be disconnected first. When installing the battery, the positive cable should be connected first.
You can try disconnecting the negative cable on the battery for a half hour and let the electronics go to sleep. Then reconnect it and start it. If the spoiler deploys again and won't go down, your new battery may have a dead cell in it.
Then as ZERACER said, take it back and have it load tested.