JL Amp and Sub install questions

It's been a while since my high school days since i've put in a subwoofer setup. 

My only real option is to keep the factory stereo (I-drive).  It is pretty good quality, it just needs some low-end. 

If I know the + and - wire colors of the wires coming out of the factory amp feeding one of the factory subs under the seats, then all I should need to do to capture those sub frequencies is to use some tap-in connectors on those to the speaker level to rca converter (in phase), then the pre-amps to the amp.  Then I wire the two postives together on the DVC JL sub to one postive terminal (output) on my monoblock amp and the 2 negatives together to one negative terminal (output) on the amp, right?  I've got a JL 250/1 and a 12w3 (4 ohms).  Thanks again for any help.

Thanks for the quick reply!  My car is an '04 BMW 530i.  It is pretty freakin chock full of electronic goodies.  The factory subs are underneath the front seats, and the only way to get to them is to take out the seats.  The wires feeding them are in a HUGE wiring harness in the trunk, i'd just assume leave them be.....who know's what i'd be screwing up then...This car also makes upgrading the stereo pretty hard.  The whole car is hooked into this computer (I-drive), and that controls everything from the Climate to the Stereo.  The stock stereo doesn't sound too bad, I think it will bang once I get some low-end.  I'll post pics when I get her all set up.....

yeah i would tap into the factory sub wires since it already has low freq running too it. but thats my opinion <diff strokes-diff fokes>

***UPDATE, need some help***

Ok, I was wrong.  The wiring information I had for the wires going to the subwoofer was for the Top-HI-FI system, not my system, the standard HI-FI system.  Different amplifer.  The Top-HI-FI amp feeds all the speakers in the car.  My low-end amp only feeds the subs.  Through my own heavy research, I do know the amplifier that is putting out the bass frequencies.  It gets the signal from the Head unit (either already crossed over to bass frequencies or not) and amplifies the signal at 100 to 500 to the subs.  Here's the catch, there's a wire harness with a bunch of wires on it connected to this pitiful little amp.  I don't know which ones are input and which ones are output.

Does anyone know how to test what's on this wiring harness?

Thanks for any help..... 

Actually, the head unit feeds subwoofer frequencies to the small subwoofer amp i'm talking about above, then the amp feeds the two subs, one under each seat.  I've looked at the small factory amp that feeds the subs and have figured out what the speakers are (I think).  4 wires are about 1.75 mm thick.  The other wires are small black wires bunched together that most likely feed the signal to the amp.  Then there's a thick red wire (power), a thick brown wire (ground), and a real thin white wire (remote).  The four 1.75 mm wires i'm talking about look like two pairs, side by side. 

When I hook up each pair to the test speaker, how do I know if i've got it in phase or not, + to +   - to -  ?  Won't the speaker still move regardless of if it is phase?

Ok, i've added a picture here to explain what I think are the speaker wires.  Going from left to right :  SMALL BLACK WIRES-input signal from head unit, FIRST SUBWOOFER PAIR-BROWN / white stripe-RED / blue stripe, SECOND SUBWOOFER PAIR-BROWN / yellow stripe,RED / white stripe, AMP REMOTE TURN-ON-small white wire, POWER-thick red wire, GROUND-thick brown wire.

Sound about right?

If so, then when I hook up my test subwoofer to each pair, how do I know it's in phase?  Won't the sub still play even if I hook it up out of phase?

I'm going to try and tap these wires and feed them to an speaker level to RCA converter which will then feed my JL 2501.  I want to have everything in phase (or does it matter?), so the JL amp is receiving pure sub frequencies.  I know for a fact from some BMW documentation I found that this little amp outputs 100 - 300 HZ frequencies only.

Thanks for any assistance.......