still planning out my system.....is there enough room in the rear deck of a 3rd gen sedan to replace the 6*9's with 8 or 10 inch free-air subs? thanks....im trying to avoid taking up the space of a box.....plus removing the rear fill should be good for imaging, right?

 

I suppose it could be done, but I am not sure that it would be the best sound scheme, and it certainly would require a visit from our friend the jig saw to fit it to the metal.
If the car is new, and has a warrenty on it, I would say bite the bullet on the space in the trunk instead. Overall I have not been too impressed with free air anyway, the whole trunk ends up vibrating, and it would require a good bit of sound deadening to accomplish it right. Between the metal metal of the rear deck, whcih is part of the rear supension support system, and the insluating of the trunk the proposition is hardly what one might call non-invasive.
You are right on with the removal of the rear deck speakers to add access for sound to the cabin, I will be doing that on the upcoming upgrade on the Mazdaspeed sedan I am now driving.
It is a neat, simple and elegant way to port the sound. There is a company out there, subsolutions, that sells a preboxed subwoofer that you install in the trunk, and then it uses flexable hose to port the sound into the cabin on the rear deck. With a little imagination, I suppose one could work out a similar scheme for a box and the 6x9 hole in the deck of a protege....
Good luck and let us all know what you do decide!
Happy Valentines to you!