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
2005 X5 4.4i
2004 E46 M3
----------former vehicles----------
1999 F250 Diesel
1998 ///M3/4
1995 ///M3