You need to take it to a dealer where they can hook an instrument to access what output code your car is giving. Depending on the code will determine whether or not it is a seat belt, steering wheel, or the air bag sensors. If no code, then maybe it is the charging system. Would not the charging light illuminate if it was the charging system however?
Somebody said that they had their airbag sensor replaced for $450. Pretty good since they cost normally around $1100 I believe down here.