I built my wall in my Astro behind the second row of seats and it sounds fine. Ive seen and heard walls in the same location and they sound good do hair tricks and do decent numbers on the meter for a daily tuning.
I am pretty sure that cochese was being very facetious with his comments. You can certainly build a wall behind the rear seats. It probably won't have the output potential that a wall behind the B-pillar will have, but it will definitely be fine to build a wall there.