I finally broke down and bought a replacement sub for my '05 AWD Limited (although it really doesn't need one), I got a 8" Infinity Reference sub (850 watts) thinking that i would keep it all looking factory. After looking at the rear panel i realized the sub's not behind a removable grill... its actually just a big molded panel. I didn't want to pull it off right away because in the past those types of connectors never go on as tight once they've been put on and off a couple times. I took the more aggressive approach of scoring around the (fake) grill with a sharp knife (i wanted the sub exposed anyways). The next thing i noticed was that the sub is round but the mounting bracket for some stupid reason is square?! (now i'm thinking solobaric might have been nice) So i pulled the four screws and fought a little with the adhesive used to secure the sub. I'll post some pics of the factory sub later. After i got the old sub out i dry-fitted the new one and discovered another problem... the mounting bracket of the Endeavor is about 2 1/2" deep on the bottom and 1 7/8" on the top leaving a big ugly gap between the mounting bracket and the panel. Also, because of the flare in the wheel well i need to put a spacer behind the new sub to give it some clearance. Back to this gap problem... if anybody has any ideas on how to fill it in it would be greatly appreciated, i've been asking around already and got a couple ideas. I went to the local audio shop and the guy was skeptical about the whole idea... he didn't think there would be enough room for the power of the new sub causing too much resonance... he said to dynomat the whole inside of the sub cavity (approx. $100), plus he said the factory amp wouldn't push it. I installed the new sub temporarily just to see if there was any distortion or rattling and was very happy to hear the sub worked surprisingly good with the factory amp... i would say its 80% better then the OEM sub and definitely worth doing (cant wait to get a amp to power it properly!) The factory system is better then alot of others without any sub!
Pictures to come...

just purchased an 04 endeavor that has no factory sub. i was talking to my instal guy. he wants to build a enclosure to fit a 10 w1 jl but we may stick with the 8. he seems to think there is enough room to keep everything factory with the 10. did you end up having enough room for the sub to fit flush? i work in a body shop and going to build the box out of glass.

i have the model with the factory sub, i was just swapping it out for one more powerful (but still 8"). behind the plastic panel in the trunk there is a mounting bracket (metal) for the sub on my endo, but its recessed back about 2 inches from the plastic, i still dont have it finished, im waiting to get an amp before i do that. you working in a body shop would make the process a whole lot easier.