The rear O2s are looking for cleaner exhaust after the cats. If you install them in test pipes, the exhaust will be the same condition at both sensors. The rear O2's are also looking for heat. Given those conditions, I had good results on a 96 by strapping the rear O2s to the crossbrace with the tips touching the downpipes...heat from the pipe and clean air from being outside of the exhaust system. Never had a check engine light.
------------------ Tada! "Low compression in cylinder 6."