One problem related to rear seat belt has been reported for the 2000 BMW 540. The most recently reported issues are listed below.
Both rear seat belts have failed. The mechanisms has not allowed them to move in or out, effectively making them useless. The first was under warranty, but no parts were in stock and we did not leave the car. About a month later, the other side broke. We had not been using the car and finally took it in and had to leave it for two weeks to get parts and have the seat belts replaced at a cost > $500.