The error for the seat comes from the fact that it thinks that someone is sitting in the seat and doesn't have the seat belt buckled. Probably a problem with the sensor mat or wiring, not the seat belt buckles. You could try doing up the buckle and see what happens.

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