I wired my car with a set of speakers and amp. No sub yet need to build the enclosure. I had to cut the wires crimp some ends and added some line out adapters. People have said to get the HU first that's where the main problem is with the WRX. The stock speakers are really not that bad. Im sure adding a sub would be similar. For my old car I just tapped into the one of the speakers. I know its not the correct thing to do but It worked.
I decided to do the speakers first since I wanted the ipod,Bluetooth, and aux inputs as well as and the HU would be more then the speakers. I already have the subs from another project that never got done.