The rail extensions hold the window and window molding in the down position.
The window rides in the rail extension in the down position. Use spray contack cement to replace the plastic (vapor barrier) when you are done.

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2009 CLS550 55,000
2004 ML350 144,000
2004 X5 95,000
2002 X5 165,000
1996 320S 155,000 (sold)
1991 190E 192,000 (sold)
1989 300SE 160,00 (sold)
1984 300D 210,000 owned since 85, (sold)
1984 300SD 160,000 (sold)