I am planning to get two of these Pioneer subwoofers:
PIONEER TS-W309S4 Champion Series 12" 1400W Single 4 Ohm Voice Subwoofer Sub:
12" Single 4 ohm Champion Series Subwoofer
Power Handling:
Peak: 1,400 watts
RMS: 400 watts
Impedance: Single 4 ohm
But I don't know much about car systems...