Get a proper scan tool so you can read ALL the codes and be able to reset the air bag light. If the "Air Bag" light is on you could have more going on than the seat sensor. The seat belt light is on as well right?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)