51 16 ... Removing And Installing/Replacing Inner Trim On Window Frame (Roller Sun Blind Guide Rail) On Left Or Right

Unclip trim (1) and pull out in direction of arrow from door trim (2).

Fig 1: Pulling Out Of Door Trim

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Installation: 

Locking rails (1) of trim (2) must not be damaged.

Fig 2: Identifying Locking Rails And Trim

Courtesy of BMW OF NORTH AMERICA, INC.