Does anyone think it's possible to reconstruct the rear speaker deck to incorporate a single sub...maybe a 12" sub? Or is there enough room in the current rear deck to cut a whole for the sub?

Definitely not a 12, maybe a 10. You will also have to get hydraulic lifts for your trunk because the torsion bars will be in the way.

Now, that I think about it, I don't think a 10 will even fit.

And even if you do, it's now infinite-baffle, which is an inferior-performance method to use for any sub... so it's kinda not worth the work.

A high-performance 8" in a small box would be more valuable than a 12" IB...

I agree--not worth the effort for the sq you'd get. You could buy a 1-ohm stable amp to drive the stock sub, but the rear deck would vibrate like crazy. I think a seperate box would be much better from a sound quality perspective.

I just took a look in the trunk and it definitely won't incorporate my 12".

If I were to remove one of the 6x9s from the rear deck, would I have to remove the entire deck to do so?

While i do agree that IB is not the easiest thing to do correctly. Hell to be honest its harder to do then almost any box but when done right can sound very good Look at Scott Buwalda's 2003 Nissan Altima it won 2 world comps for SQ with his DLS sub in IB configureation.. not only did it sound good but hit really hard when wanted above 140.. http://www.meisearch.com/share/projectaltima/ here are a few pics of his car pics 20 and up will show the subs..

that's exactly I want to do with the subs, but with litttle room, it doesn't seem likely.