I wouldn't use car stereo subwoofers in a guitar rig. They use too much wattage (A POS Dirt-cheap one runs at 150W RMS @ 8ohms) and their frequency response is better suited to a bass guitar. Plus car-audio speakers are only designed to work in enclosed cases (or ported) and it'd be foolish to put them in a free-air cabinet (All guitar cabinets are free-air cabinets) because it reduces their wattage handling and nobody can figure out an equation on how much since it varies indefinitely (between speakers).

In a bass cabinet built to the proper size (Recommended by the speaker manufacturer, usually 1.5 cubic feet per subwoofer), it might not be a bad idea if you could find an amp powerful enough to drive 4 of them; I always wondered about hooking a mono-bloc amp to 120AC and seeing if I could use it to power a home-made cabinet (you'd get no EQ though).