Also, some stereos differ in their front/rear outputs. My old Mazda6 had a very flat signal on the front speakers, but was quite different on the rear signal (confirmed by some online research and just listening). When I hooked my subwoofer up, I spliced into the rear speaker wires and it yielded almost no bass, but changing to the front speakers made all the difference in the world.
But yeah, you definitely have them wired wrong.