This has been a long-running problem with many models of BMW. On the new cars, the plastic blanking plate can be replaced without too much trouble, on older cars it can be more tricky.
What is important is that the car is thoroughly dried after it's repaired, as many electrical systems reside under the seats and become submerged. These are fairly waterproof in the short term, but if the car is left wet, it will lead to difficult and expensive problems later.
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