Should I insist that my 2009 Ford Focus's misted-up rear light clusters are corrected at service?

I own a 2009 Ford Focus with 5,000 miles on the clock, garaged when not in use. I noticed condensation inside the two back light clusters, and later took the car to the dealers who said they replaced them. The condensation has appeared again, and I happened to notice a Ford in a car park with similar in its light clusters. My previous Astra suffered from this in a front headlamp unit which eventually dimmed the reflective silvering, so I am reluctant to just ignore this. The car is due for its 12-month service. Does this condensation matter? Am I being picky to ask for this to be corrected? Is this a Ford problem?

Asked on 13 November 2010 by KE, via email

Answered by Honest John
It's a problem with large polycarbonate mouldings. They expand and contract at a different rate from the surrounding steel, so the seals break. Could be worse with the even more complex mouldings of the new Focus.
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