I have one in the passenger footwell. No extra 12V battery required. It was too loud, but I just turned down the gain.
I like it quite a bit actually.
I added one because the stock subwoofer kept cutting out at higher volume. Driving with the top town on the highway and listening to music was useless.
Aftermarket sub and bypassing the stock subwoofer via the aftermarket head unit crossover controls solved all this.
I'll find some pics and provide the subwoofer model number when I have time. It's a very small "flat" subwoofer and I highly recommend.
Stock subwoofer did not work well for me at all.