Thanks so much for posting this info. I have a European spec BRZ which also has tweeters in the dash, woofers in the doors and mid-range speakers in the back. I thought all versions were equipped with an amp, but apparently this is not the case.
Yesterday I installed Alpine two-way speakers in the front:
http://www.crutchfield.com/p_500SPS6...10.html?tp=105
My idea was to disconnect the factory tweeters (which are too 'bright' in my opinion) and use those in the woofers. However, after disconnecting them I lost the sound in the woofers too. Your post explains why this happened.
My question is how to disconnect the dash tweeters properly. I would think that simply connecting cables running from HU to those going to woofers should work. Are there any risks?