It's a ski bag. Allows you to put your skiis in from the rear and not have the snow melt and get over everything. Really an awesome feature I think (although I assume most people would the skiis on the roof).

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Radelow
2011 X5 35d
2011 335d
2007 F150 5.4 4x4
Factory Five Cobra
----------former vehicles----------
2008 E60 M5 ... 2005 X5 4.4i ... 2004 E46 M3 ... 1999 F250 Diesel ... 1998 M3/4 ... 1995 M3