WARNING: This page is about a different car, the 2025 BMW M4, 2024 BMW M4, 2023 BMW M4, 2022 BMW M4, and 2021 BMW M4. However, it is still accessible from the selected car via links, so may be relevant.
Open up the floor trims in the area (1) in the arrow direction.
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Turn the sealing cap (1) in arrow direction (counterclockwise).