Thanks, it is almost there.

Yes, you can do just as you are suggesting. It is called free air or infinite baffle sub-woofers and you may want to check out the JL audio IB4 8" which should drop right in your factory locations beautifully. BTW, you have four holes so you will have to use two or four speakers because they are 8 ohm single voice coil. Two drivers will give you a 4 ohm load and you can run a 150-200w RMS mono amp on those. If you choose to use four drivers, you will get a 2 ohm load and can use a 300-400w RMS mono amp.

http://www.jlaudio.com/subwoofers/pdfs/8-12IB4_BDS.pdf

If I did not build an enclosure, I would have gone the four driver route. It is my understanding that in an infinite baffle application, the bass is not as tight and accurate.

I would suggest buying component speakers for your front doors. You have two factory choices for your seperate tweeters. The a-pillar or the dash. Mine are in the a-pillar right now, but I want to move them nearer my mid-bass in the factory door location. If you know home audio, you know about imaging.

Coax speakers tend to have lower quality tweeters and a high pass filter as opposed to a true cross-over network. They would be fine for the back doors.

If you change speakers, you will have to change at least the amp. The Bose P.O.S. will not drive aftermarket speakers. It is intended only for the Bose speakers.

You can keep your head unit as I did but will need a high level converter or high level inputs on your amp. I thought this was a good idea but have changed my mind. The sound quality of the Becker head unit is dismal. I am trying to decide what type of aftermarket head unit to buy. I am leaning toward the Nakamichi CD-45z because it looks quite good with the Mercedes dashboard as it is available with amber illumination by special order. http://www.nakamichi.com

Door panels are easy to take off once you understand that you will have to apply some pressure to get the job done. I was reluctant at first but after a pulled hard enough, it worked fine.

Changing the speakers, HU and running new wires can be accomplished in a day. Everything is well put together and easy to take apart. Building the Sub enclosure takes more time. I am not sure about removing the rear deck to get to the sub-woofer speakers but if you ask your dealer how it is done, they should be willing to show you the directions on how to do it the right way.

Good luck