High/Low Level Converter

I have a Chevy with a Delco CD player.  I've heard it has some sort of small built-in amp for the door speakers, and I think it does, because they sound better off the HU when I have it hooked up, rather than my Kenwood HU.  My question is, if I get a High/Low conveter so that I can hook my amp up for subs with this factory HU, can I just tap into my rear speaker lines, or do I have to use those wires and disconnect them from my rear speakers?  Also.. (I guess you can see I've never hooked an amp to a factory HU).. what do I do about a remote wire for my amp?  Should there be a remote output wire on a factory radio, or is there one I can use?  Thanks.Donnie

nevermind about the remote wire...i'm an idiot...ACC wire...DUH!  anyway.. if i just tap into my rear speaker wires for a LOC..will that be like running speakers in parallel and send a load of too low impedance to my HU??  if so.. is there anoyher option so that i can keep my rear speakers and hook up an amp to subs??Donnie

they have one at circuit city in the car audio section for about $20.. but i dont remember the brand.. is that about the right price for a decent one?Donnie

Soundgate, PAC, or audiolink are all good ones