how to wire sub/amp to 4 speaker factory

I'm trying to wire an amp and a 4 ohm sub into my daughters 2004 PT Cruiser. It has the factory 6 disk radio like the RBQ version here http://www.jeepworld.com/accessories/liberty/stereo.htm It has two dash speakers (tweaters maybe) two speakers in each front door and two speakers in the rear pillars. I've not taken anything apart yet s I don't know the factory speaker ohm's.

I assume the factory head unit doesn't have a subwoofer connection, so what do I need to do to connect the single sub to the head unit. I assume I'll need a line level adapter if the amp we get doesn't have it built in. It's just the wiring of a single sub into the exsiting 4 speaker system that I can't seem to grasp. It seems like it would throw the ohms off on one side or the other. Or does the amp correct for all of that and make the sub essentially transparent to the head unit.

Use an LOC like THIS one connected in parallel with the rear speakers.  You can connect it directly to the speakers so you don't have to remove the head unit, and it connects to both left and right.  It will not interfere with the operation of the factory head (although by adding a sub you are likely to suddenly be disappointed with the performance of the factory system.)

Thanks, thats what I was hoping. I just couldn't find a diagram or post anywhere that confirmed it for me.

O.K. the Line level Adapter and the dedicated amp handle all the speaker wiring worries I had.

The amp I've ordered blocks frequencies above 300 hz or above 55 depending on how you configure it and only sends out to the sub anything below that. I'm going to put it at the 300hz and below setting.

What Id like to know now is how do I figure out what value capacitor to put in line with the other speakers to only allow frequencies above say 100 hz to go to them. I picked 100hz out of the air because I figured the factory speakers could perform well enough down to there. If a lower cutoff would be better I'm o.k. with that to.