After reading everything about subframe connectors, strut tower braces, and wonder bars (steering box braces), I was wondering if there was also a lot of flex behind the driver's seat. It seems like the only bracing is the C pillars, but there's a lot of chassis behind those.

 

You're talking about side-side flex as in a road racing application, right?

I would guess there is some but not as much as the front, considering you don't have as much weight, you have sheet metal spanning the rear frame rails, the roof and quarter panels, and you have the rear sheet metal where the bumper/taillights reside.

If you think about it, the front is a lot more "out in the wind" as far as support goes, which is why you need the wonderbar and STB to tie it together/keep the geometry correct.

If you would like to do something for the rear, like a STB, remove the rear seat belts and fabricate a bar that spans the area between the rear seat belt mounts. There is a nice solid bracket that's welded to the inner fenders and has a beefy bolt that you can use to secure your bar.