Don't post on here much but I thought I'd help as I've spent a few years back in the day wiring up ridiculously loud car stereos!
You won't be able to run these off any existing speakers. What you need is a line out off the head unit or connect a small adaptor which will take the signal off the rear speakers and convert it to a line level signal which you would present to an amplifier. Alternatively some amps have a speaker input which gets the signal from the speaker feeds. The best way in my opinion is to take a line out of the headunit and take it through a crossover or into an amp with a low pass filter etc. Then you wire the subs to the amp. If there are 2 of them you will need to work out the best way to wire them to the amp. Ie parralel or in series but that depends on the amps rated power / resistance compare to the speaker drivers.