Surprise! The two speaker wires are not right/left but one side door speaker and opposite side tweeter channel. The factory subwoofer is powered. The factory radios have no way to adjust the subwoofer frequency (there's no actual crossover, just additional sound) or provide any real power.
Are you planning on putting in your own mixer and amp?
If you'd like the guts of a subwoofer, I have one I intended to use for an audio-check setup for the $80 Radio10s I provide. The subwoofer housing got damaged in transport (small chunk of plastic broke off) but the guts are fine, I'm sure.
The power versus response, on the other hand, may be way off with no way to adjust it when coupled with a more efficient cone perhaps with different impedance.
If you're interested, shoot me a private message.