Hm, maybe the wood trim is prone to stress-cracking from the impact of opening and closing the door? That would explain why it's always the driver's side door. One could do some research and see if X5's in the UK have the issues on their driver's side.

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2003 X5 4.6 | 2000 540i Touring

Formerly owned:
2004 Range Rover | 2000 M5 | 2000 540i/6
1995 750il | 2003 Mercedes S500 | 2003 540i/6