You need to remove that plastic retainer at the base of the pillar/shoulder-belt cover - the one held on with two philips screws. Then you can drop the pillar cover enough to allow it's metal tab to re-engage the slot in the upper part of pillar. The pillar cover slides together at the top with an up & in motion. Then the plastic base screws on to secure the cover's base. As the previous poster mentioned, you may need to re-bend the cover's metal tab slightly so the upper end of the cover will grip the pillar tighter.

Happy Motoring, Mark

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DrDKW