Stereo & Electronics - Question about adding a sub to a Monsoon system

If this has been asked a million times I apologize, I just did a seach using the "search this forum" link, but didn't find what I was looking for.

I have a box with 2 Polk 10" Subs (forget which model right now), and a 600W Kenwood stereo amp. I used to have them in my Grand Prix (daily driver), but as I'm now selling that car, and can't put that box in the Dakota, I've decided to take 1 of the subs out of the box, make a custom driver's side "stealth box" for it, and planned on using the Kenwood to power it (a bit overkill, but I already have it, so it's cost effective).

How would I go about tapping into the stock T/A (WS6) Monsoon system for the sub?

Quote from the FAQ:
Adding a sub to the factory system is relatively easy. The most common method is to add an amp and mount a stealth box with a 10" sub in the cubby space on the left of the t-top storage area. You will need a line-level adapter that is available for just a few bucks from any stereo or electronics shop. It will convert the speaker-level output of the head unit to the pre-amp (line-level) input needed by most amps (you can also buy some amps with speaker-level inputs). A common place to mount the amp out of sight is with a mounting board at the back of the car below the hatch lock. There are all kinds of options if you want to show off your amp - check out sites such as subthump.com for some ideas.

to give a more direct answer, theres two places to consider getting the signal for the amp.
1) remove the deck, get normal harness and a reverse harness, and connect the line-level-converter behind the deck.
BENEFITS: locates the RCA behind the deck for when you change the deck. Also gets an unaltered signal. Also you dont end up modifying ANY wires in the factory wiring.
DRAWBACKS: more work, longer wires needed.

2) tap into the wires going to your rear speakers. you need and left and right to get the full mono bass.
BENEFITS: fast and easy. less wires needed.
DRAWBACKS: that signal is crossed over an might not have all bass frequencies. also the sub will fade with the rear speakers.

i usually get it behind deck.

just get a line out converter and tap in behind the stock radio i have done this a number of times with the monsoon system. either just strip the wire and tap in or get splice connectors

You can also connect your LOC at the input wires of the Monsoon amp since it's exactly the same signal as behind the head unit. It's really just a matter of convenience and personal preference. Although, as fast01z28 mentioned, if you're planning to do more upgrading later then it wouldn't hurt to already have the RCAs going to the dash.

Thanks for the help.

By "rear speakers", do you mean the hatch speakers, or the sail panels?

I'll probably go that route at first since it costs less and would be quicker, but eventually I'll probably get around to doing that line out converter stuff (and am favoriting this thread to keep it handy hehe).

Anybody here make their own d/s stealth boxes? I'm tempted by the ease of just buying one, but I have piles of scrap wood and am on an extremely tight budget right now and think I should at least try to make one first and see how it goes.