I've got this in my STI, and it'll probably get moved over to the BRZ. It's more than enough for average listeners. I'm into heavy bass and will probably need to get another amp and a 12" to cover the full range of frequencies. Best part of that amp is it's tiny and doesn't get really hot so you can mount it just about anywhere. It'd probably be perfect in the spare tire compartment.
http://amzn.com/B0068XYNH4
http://amzn.com/B004T163PW