I bought a crashed 2005 Cadillac STS. Fixed the body, replaced both front airbags, module and had both seat belt buckles rebuilt. "Service airbag system" message still appears. What else could be triggering this MIL?

Also, I too just replaced the battery after it went dead, and now the radio will not turn on (non-navigation); I read an answer about checking the fuses under the back seat, but why would the fuse blow from just a dead battery?
thanks in advance,
John