I finished my amp & deck sub install.

I use a 150 watt mono amp & an 8" free air sub with a large amount of deadening material. Most rattles were trunk related and took some additional closed cell sheets of foam.

I used a few sources as my reference material:
http://www.mazdas247.com/forum/showthread.php?t=123698578

For the aux input this thread was awesome and works great (remember, this is just a stereo audio feed, not integration) I didn't follow this exactly...as removal is always a plus.
http://www.mcx7.com/showthread.php?t=1495

As for the amp, it is actually mounted on a rack attached to the braces in the back behind the seat. It is actually impossible to know it is there without being told because no factory pieces were removed to make room. It is simply in the void behind the seats.

I actually unwrapped the audio feed from the factory bundle to the rear drivers side and that is where I spliced a set of RCA's onto it. I also put RCA's on my bose sub harness if i ever put it back in. For the love of God, STOP PUTTING LOCs on Bose systems if it is not a speaker output line.

As for the step down to an 8" from a 9.5" sub...whoopie, the bose sub puts out effectively zero sound. The 8" hits harder, actually makes sound and helps the car stereo tremendously. Its not a ground pounder, which wasn't the goal, but man does it sound much nicer.

I set myself up on the power side so I can do the amp under the passenger seat at a later date for the doors.

I will post pics in short order.

I agree that the factory (not just Mazda) should make the audio wiring more universal, but there's no reason to in their eyes. I'd like to see the pics of the progress and finished product.

BTW, if you're ever around Longmont, stop by the dealership and say what's up.

I stripped the signal wire out of the factory wire bundle and then spliced on RCA's behind the bolster. Ultimately, this route looks like crap and introduces too much unnecessary wire.

Instead, I added RCA's to the signal side of the amp under the passenger seat and ran the RCA's down the right side to the amp.

I plan on replacing the door speaker amp eventually so this made logical sense anyway.

So far, much happier with the sound of the car overall. Just be prepared for a large amount of deadening material for a deck mounted sub with any reasonable size driver.