Holy Crap! You might just be a genius! I'm new to the forum so let me give you some background.
I purchased a 2011 Subaru WRX Limited in December of 2010. The car was technically used but only had 1300 miles in it. The guy that owned it for a month and a half installed a Pioneer AVIC-Z120BT, backup camera, dynomat in the doors, and some miscellaneous other things. To get the steering controls to work, I later found out that he had a PAC unit installed (that's the little blue box, right?).
Anyway, just as you mentioned, I've been having the same problems and it's been driving me crazy!