Many Mercedes' built between 1997 and 2004 have this problem. It was a period when Mercedes-Benz made some significant errors in sealing and preparing its bodies for painting. The company seems to resist rust claims on cars more than eight years old and legal action against it is likely to be expensive because (as far as I know) there had been no significant UK judgement against the company over this issue. Happily, the company is now building excellent cars again, but this will not appease owners of rusting 1997 – 2004 cars.