Welcome to X5World! :welcome:
This happened to me a few months ago on the rear passenger side window. I do remember having a hard time getting the panel off at the door handle. If I recall correctly, I tilted the panel counterclockwise, played with it a bit, and it finally popped off.

There's a metal clip there behind the door handle that is tough to unhook.
Sorry I couldn't be of more assistance.
All the best, Zach
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2005 Z4 3.0i, Sapphire black over Dream red, 6 speed, ZPP, ZSP, heated M-sport seats, Dinan upgrades (ram air duct, high-flow intake, free-flow exhaust, strut and supplemental braces)