Helpless with a 2010 Yaris sedan

[Meechity]

I know next to nothing about cars: 1)they roll around, and 2)I'm stuck in one for 3 hours daily. I just got my front and rear speakers replaced and will soon be getting a small 4-channel Rockford amp because the new speakers have literally done NOTHING to improve the sound (which is hollow and tinny). I'm almost completely convinced it the head unit's fault; however I was told that it's basically impossible to replace it unit without basically destroying the dash or spending thousands of dollars. Here I've seen photos that say otherwise.

MY basic question is: what head unit will fit in my car and cost between $300 - $500? I have a 2010 Yaris 4-door sedan (NOT the sport model) with a simple audio jack (no iPod integration).

:) Virtual thank-you candy to anyone for your help. I'll keep browsing the forums, though it's very hard for me to follow the technical specs.


EDIT: This is what the center part looks like, with the stereo and knobs and such.
http://www.yarispartsstore.com/image...blue-sedan.jpg