If you watch Ebay, someone puts these subwoofers up from time to time. It might be possible to buy one from your dealer, but I don't know a part number. If anyone does, please post.
The big question was, is there room for it without cutting the non-monsoon plastic panel behind the seat. The answer is no. Some cutting is required, but probably not as much as I did.
First thing is to remove the door sill covers, which just pull up, and the corner panels that have the rear speakers. These just pull away, but some of the clips may stay with the car. If they do, it's no big deal, just pull them out with pliers and reinsert into the cover.
I went ahead and removed the seats, just for the easier access, and for running the amp wiring under the carpet. It only requires removing 4 nuts each and unhooking the seatbelt sensor wiring harnesses.
Next remove the two 10mm bolts inside the waterfall compartment, in the top corners. NOTE: As Tweety and many others have warned, the nuts on the trunk side are not secured, and must be held or duct taped to prevent dropping down into a hard to reach area.
After this is done, the back panel just pulls off. I worked top to bottom, just pull straight towards the front of the car.
I was surprised that a couple of the connection points had no clip but had some double sided tape across the clip hole, with the protective clear plastic still on the interior side. Not sure what that's all about, but I left it as is.
In the next pic you see that the mounting studs for the factory amp are still there. As there is no wiring harness for the factory amp, I don't see how you would hook one up, but if you want to try, at least it would be easy to mount. I had an old single channel amp lying around, but it is way bigger than the factory one. I checked that the studs weren't holding anything on the other side of the wall, and when I had satified myself, I cut them off with a dremel cut off tool.
This is the point at which I asked myself, "What have I gotten into?"
Here is the amp mounted and wired. There is a nice unused grounding nut right above which would normally be used for the factory amp. There was no interference with the map pocket on the drivers side, so unless your amp is extra large, you should have no problem there.
The next pic is the sub in place and wired. You will need some washers and some metric nuts. If I remember correctly, they are 8x30's, but don't trust me.
The sub is a dual voice core 2 ohm x 2 ohm. I had tried to figure this out before by removing the speaker and looking for info on the back of it. No luck. Then, I finally noticed a sticker on the back of the plastic enclosure. Sure enough, there was the info. As my cheap amp was only stable at 4 ohms, I hooked the sub up in series for a 4 ohm load. If you use a 1 ohm stable amp, you can hook the sub up in parallel. For more info on this, I found this nice writeup. http://mobile.jlaudio.com/support_pages.php?page_id=161 The light blue wire is the switch wire from the radio to the amp. I left the extra slack in it as I plan to mount a switch next to the parking brake to turn the amp on and off.
As I tried to measure interference between the back panel and the sub box, I thought the whole lower section of the map pocket was going to have to go, so that's what I hacked out. It works o.k., but I plan to work on it some more, opening it up over the entire speaker and installing a grill, plus maybe making the pocket functional with some speaker cloth applied over the back of the panel.
Now that I can see it all in place, I'm pretty sure you wouldn't have to cut as much as I did, and could probably get it to work just by cutting out the exact speaker circle. Of course, you also have the option of buying the entire monsoon backpanel, but the paint matched waterfall portion complicates that.
Anyway, I hope this helps those of you who might be interested. I know you high end guys are probably going to build your own sub box, and you're probably having a chuckle at my wal-mart amplifier, but all I wanted was a little fuller sound and I'm real happy with how this sounds with my Pioneer headunit.
man, thats intense. there is no way i am gonna do that by myself.
Do you want to come up to Canada and do one for me
And PS: thanks for this post, many have been wondering
I've been talking to one of the Big Box type electronic store and the kids there that are familiar with the Kappa series are suggesting that I install the Kenwood 6-1/2" 150-Watt Powered Enclosed Subwoofer (Model: KSC-SW1), as everything is enclosed in a small package.
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TravelinRaider,
"Neeser, the Green Eyed Lady"
(Envious, w/auto - hope it last as long as my 1974 Bride)
Looks really good. I wish I didn't have the moonson back panel. As I want to do something custom back there. I would put it behind plexi glass or something like that. Right now i just have the moonson grill. Not too much show for it.
Looks good. Keep the cutting going. Before you cut. You should've put the sub on the inside of the panel and traced it out, so you cut it out from the back and not the front. Still going to look good none the less.
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Performance Auto and Sound
Compression Performance
One thing I'm not clear on is why people mount the subwoofer behind the passenger instead of the driver. It seems like it would make more sense to put it behind the driver so that they can feel it.
Pangloss, the reason I put it behind the passenger is because that's the only place the factory sub box will fit. It molds back into the nooks and crannies to get as much cubic footage as possible.
Even if it didn't, I think I'd still put it behind the passenger side because I can slide the passenger seat up a little to let out some sound. I drive with my seat all the way back.
As far as the cutting, Compression, trying to put this big box up against the big panel, outside of the car, and figuring out where they will intersect requires a lot more patience and measuring than I was willing to deal with, and I kind of rushed that bit in an effort to finally get my car back together. Then, when I was done and turned the key, the car didn't start. Turns out, leaving the door open for a few days will run your battery down, who wooda thot?
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I added it to Rodeo's "how to" 
Turns out, leaving the door open for a few days will run your battery down, who wooda thot?
Just a notch or two is all you need 