Underspecced plastic materials :-( The dark-tint/smoke-effect rear lamp clusters seem to have this problem more than the others. Replacement P/Ns for the European-market taillamp parts that seem to be less prone to melting are:

A202 820 01 66 (lens, left)

A202 820 04 66 (lens, right)

A202 826 04 91 (lens seal, LH = RH, need two)

A202 820 01 28 (housing, left)

A202 820 02 28 (housing, right)

A202 820 01 77 ("lamp support" = bulb holder assembly, left)

A202 820 04 77 ("lamp support" = bulb holder assembly, right) 

The least melt-prone items are the ones with unsmoked red lenses and amber turn signal lenses. They're usually tough to buy through official channels in North America. Ebay to the rescue, for example item number 250156969410 . This particular auction specifies shipping to Germany; I've found Germans generally will ship to North America if asked and properly paid. If that auction doesn't work out for you, here's the search string I used to find it; keep an eye on it and eventually you'll find a "juicy" pair of lamps to buy:

http://tinyurl.com/27snj7

Best of luck!