The 2009/2010 LCI tourers have a known defect with water ingress into the rear light cluster which makes the LEDs fail bit by bit.
They changed them all at service in the US, but not here of course.
My N/S one went when a year old, was changed under warranty, but they wouldn't change the other one even though it is a known issue, so now i'm waiting for that one to fail....
It's a sealed unit, £175 for the lot if it fails.
E63 AMG here at last!