Looking to upgrade the stereo but don't want to stray too far from stock.
What is the huge space behind the drivers seat good for?
Amps? Speakers?

 

you can use the space behind the seat for either amps or speakers. Therer are tons of threads in here that show both options.

 

Amps and other electronic equipment. Do not put a sub back there unless you have the convertible.

 

One of the first setups I had used the stock radio with HLC converter to an amp mounted in the cubby you are referencing. The little mono amp powered a 10" sub in an under-strut box in the rear. The whole setup was very affordable and added some nice bass..perhaps this is a route you may choose as a first step..then later upgrade the components and H/U. The only tough part of this installation is running Power wire to the amp, but it's not that difficult. For many people doing things in stages is more affordable and allows them to stop upgrading when they are satisfied.

 

I used my for amps and power distribution and used an under strut box for the subwoofers. Free air subwoofers lack the power of a sealed box or even an honest ported box subwoofer.