I am certainly not an authority on this but I do know that they came with rear side airbags but to activate them it was a $300 (I think) option so there should be a way to activate them. It was always strange to me that BMW would go to the expense of installing that airbags without activating them, all to try and get a few bucks out of the buyer.
I could be totally wrong on this but I think there has got to be a way to activate them and clear the error code.