Hook the subs to your rear speaker wires, but disconnect your rears first. It'll do no good splitting the power if you want bass.
It's not going to be very loud, almost not worth the work. You could just wait to get an amp.
If you had the subs and rears hooked up, instead of having (say) 20 watts going to the rears, you'll have 10watts going to the rears and 10 going to the subs. If you disconnect the rears you'll send 20watts to the subs, so.....
