You need a standalone subwoofer amp. Basically you have line level outputs from your receiver, which provide the signal (which basically tells the speakers what to do) but do not provide any power so that the speakers can do the job.
The only way I can think of for you to do this cheaply would be to hook up an old receiver or something to those line levels and plug the subs into the speaker outputs. HOWEVER, this is a VERY BAD idea, because it WILL sound like ass and you will almost certainly fvck up your subs due to lack of a crossover in the old receiver, not to mention that an old receiver probably won't put out very high wattages. You would also have to pay attention to the resistance (ohm) rating of the subs and make sure that it's within the rated spec of the old receiver.
Basically, this would be very impractical. You'd be better off buying a standalone powered HT sub that's designed for home use, rather than jerry rigging car speakers for it. The HT sub will sound a lot better, be a lot cleaner, and much safer.