Have it scanned for fault codes and see what comes up. There may be a seat belt retractor deployed. If you're lucky the code for the deployed air bag is the only one present. In that case simply clearing it will turn the MIL off.

FWIW, later model MB's don't require ECU replacement when there's a deployment.

Also, junkyard ECU's will need to have the current VIN written in or the MIL will stay on.