Ok I am just thinking out loud here guys, so just help me.
I get the feeling (looking at the wiring diagram of the alpine upgrade) that the wire connecting the amplifier to the head unit just tapps on the high level signal output from the head unit. The amplifier then converts it to low level and then after amplifying it, feeds it back to the two subwoofers and two midbas and two tweeters. The high level to low level conversion takes very little of the power output from the head unit. There is plenty left over!
What one could do as an extra gimmick is to parallel tap the high level output from the head unit and take it to two extra speakers (one on each side) and mount these speakers on the extreme back of the car making it in effect a 10 speaker system. You don't even have to buy the speaker, just use the front door speakers that you removed to mount the alpine kit speakers. We have a kind of a "hybrid surround sound" without the centre channel. The question is where to mount these rear most speakers in the touring model?