The strip along the bottom edge is molding
Is that the piece that you mean is not staying fasten down
That should have screws thru it, not tacks
You could try some , screw in zinc anchors,
Put the anchor in the wood , place molding over the anchors, put screws thru molding into the anchors
No more than 18" apart, a box of 50 might do both sides
I can explain it to you.
But I Can Not understand it for you !
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1997 F53 Bounder 36s