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idk how far you got on this, but i wouldnt even worry about the far back pillar, its filler for the 3rd row and no real sound can come from a tiny little speaker like that
upgrade the door speakers either 2 sets of components or at least one set of comps in the front doors and a decent set of coax in the rear doors and stick an amp on them, will be miles above the stock sound, and you wont ever hear the filler speakers from the back
i left my bose speakers in the rear door but replaced my front door with a set of infinity kappa's component system, wasnt the perfects but one step under, i forget the exact model number but i love them