Mmm, well try removing the n/s interior boot trim and with a good light on the outside (ie daylight) see what you can see. My guess is a whole lot of daylight.
Mine has a good hole there and immaculate at the C pillar to door weld whereas yours has a lot of rust on that weld seam so you may draw your own conclusions. However each BX seems to rust a bit differently so whilst there are certain known areas, other parts are more random.
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