I learned this from the E39 5-series forum...

- If you decide to use a short piece of broomstick or wood, cut a small groove so the glass' bottom end sits on it.

- Then zip tie the broomstick so it does not move around.

- This is a temp fix, although for some people, it is a permanent fix (solo single people that never open the rear window)...

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT