Well using that wiring you can attach a micro amp that will push 50w to the door speakers and leave 100w bridged for a sub all using stock wiring.
http://www.crutchfield.com/p_500KTP4....html?tp=35782
http://www.crutchfield.com/p_020XC14....html?tp=35782
Or if you just want to retain stock speaker wiring you can run a power cable and RCAs while attaching a full size amp. Plus you now have access to high level inputs if you dont want to run RCAs and a remote turn on cable. All without cutting and splicing.
I used the BRZ wiring manual found here:
http://www.ft86club.com/forums/showt...ht=shop+manual