Good questions. Unfortunately I cannot claim any ownership over the following research, work or answers. You can thank Geeman for supplying his 2003.5 Sedan to El_duderino to run some analysis on. Here is a link to the thread that answers your questions.
http://www.g35driver.com/forums/show...08&postcount=1
I have also placed these pictures and el_duderino's comments on another server, in case the one linked to in that post goes down.
The head unit has a + and - pair of wires for front left, front right, rear left and rear right channels. 4 channels total. From these the Bose amp creates additional channels when necessary (such as the for the sail speakers in the premium coupe and the sub channel in the premium sedan). The front "components" (term used loosely for the stock speaker crap) are seen as a single channel & load to the Bose amp.
I don't know if the Bose amp actually sets up any crossover points, but instead places its own predetermined EQing on each channel it outputs- theoretically making each set of speakers sound "optimum" in the car. The front "component" sets are not actually crossed over either. Instead, a "full range" goes down one pair of wires to the door, where the wire splits to the tweeter and woofer (hence why the stock Bose amp seems them as a single channel/load). The tweeter has a resistor on the back that acts as a bass blocker.