Has anyone done this?
On Crutchfield, there are Rockford 6-3/4" subwoofers. You could buy speaker adapters for 6x9 openings, mount them in the 6x9 positions, and put a set of 6-1/2 comps or what not, in the rear doors or something, near the bottom where the only possible space would be. Put a set of Infinity Kappa 60.5cs in them, and i'm sure there would be nice sound quality. You could run the full range wires to the back doors, and the sub wires to the old 6x9 positions. Seems like it would be realitively easy, the hardest part being the door mounting.. You would probably lose some of the audio spectrum though, in what the 6x9'* produce, and they need the trunk in order to do it efficiently, so this might not be all that feasable but it would be neat. Someone should try it out and see what happens.
I'd also love to get a set of 6 1/2'* up front without chopping up the internals of the doors.
Did you know that some older (1994 or so) BMW'* only have a set of 5 1/4 inch speakers and that is in kick panels in the front of the car. They are the only speakers in the car. Imagine how poor the sound is in there! I could be wrong though, but crutchfield said thats all the car has, i forget exactly which one it was but it did say that.