Yes you nailed it - without the ski pack the tack welds will break free with chassis flex and you will hear that cover plate start to rattle. Removing the plate behind the armrest cover solves the problem. On squeaks I have now are from the cover above the instrument cluster and associated to the blower fans in the dash.

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2007 Kawasaki Ninja ZX-14
2006 BMW 750i
1972 Dodge Charger
1968 Dodge Charger
1967 Austin Healey Sprite