I bought a set of used TC door speakers to place in the rear speaker area. It took some work to get them in, but the sound was worse, not better. None of the bass you normally hear from the door speakers. My guess is that the stock system sends more power to the door speakers (where its needed for bass response) and less to the back seat speakers. I was disappointed, but thought I'd go ahead and post this in case there are others out there that considered trying the same thing.