Ok, I see where you're talking about. I've got a Zapco DC750.2 in that spot, which is 23" x 7.5", so you've got room for a pretty good amp there. From the sounds of what you have right now, you can fade front to rear, but you have mono sound, no L/R. That would drive me nuts, but to each his own, right? I don't see a need for rear speakers, especially in this tiny car, I think you're much better off having a properly set up front stage.
You didn't mention a pricerange.
The Soundqubed S4-100 is 100x4 and is less than 8"x 8", this should fit. $169.
The Skar SK-M400 is 100x4 and is 7.9in x 4.6in. $129.
The Skar SK-M900 is 55x4 and 300x1 (450x1 @ 2ohms) for sub and components. 12.4 in x 4.6 in and $150.
Skar has a ton of other amps that will fit in that space, too, go check 'em out.
www.skaraudio.com
I'm sure there are tons more out there, some higher end, but these two budget brands actually do make good power in a small package at a very, very good price. Honestly, if I end up replacing my Zapcos, if I don't go with another Zap, I'll probably do one of these, or Wolfram, but the Wolfram doesn't fit in that space, I think it's too wide.
The
Zapco ST105.D.BT will fit there, too. Also has a sub channel.
The Zapco ST-4XII is 50x4 and $199 and will fit, also.
The Zapco ST-104D.BT is 100x4 and $299, but is TINY.
Those are some options.