Re: Modifying the stereo with independence swith.
Should not be an issue and I think one person did make a sub to fit in the space, but I can't remember where or the thread name.
If you just made a box to fit the same size as the styrofoam that's in there, you could easily make one. I believe there is more than enough room for a 10 or 12 shallow throw sub.
Good luck!