converting

You said your subwoofer is powered by electricity so I assume there is a built in amplifier that is powered by 220 or 110 volts depending on what the mains power is in your area. Now you need an inverter to convert 12 volts battery power to 220 volts. You need to get a signal from your car radio to be fed to the sub woofer. It will be good if there is an audio out or subwoofer out from your radio but if not you may have to get a signal from the speaker out but it has to be to a minimum level, you need the help of an electronics technician for this. You can not get a signal from 2 input jacks, it must be output to the subwoofer. If you have time you can surf the net and get a detailed schematic diagram on how to connect things together.