Umm...I am thinking high to low level converter to make Rca out of a stock deck.....then an Amp...then the subs. Or maybe a cheap amp that has high level inputs. Best go to the local stereo shop and ask them what they use to do the same, they may have a cheap answer.
http://www.caraudiohelp.com/car_audi...dio_system.htm
on this page, look at the second pic down..that is an exapmle of a low level convertor....from this you will do the install just like your car pc