Blaupunkt THB 200C sub. Fits under seats and can take both speaker and line-level input. Will throw in an audio cable if you want it.
compact powered subwoofer;
8" woofer with polypropylene cone;
70 watts RMS amplifier (peak power: 140 watts);
variable low-pass filter (40-120 Hz);
phase switch control (0-180°);
frequency response: 30-120 Hz;
preamp- and speaker-level inputs;
9.8' power cable included;
dimensions: 14"W x 3-3/8"H x 8-15/16"D.