The last time i semi-replaced the a bose system was in my '01 Benz E430. I utillized the factory head-unit and also the factory rear door speakers. What i did was took out the front door speakers, installed a set of Diamond Hex mids in the doors/ tweets in the dash, and two JL subs in the trunk. I clipped the wires coming out of the factory amp goin to the highs up front and to the two rear deck subs but utilized the already in-place front door highs/mids wiring which saved a ton of time. I used an Audio Control LC6 line converter to wire in both my Zapco studio amps. Sounded amazing compared to the factory setup. Front highs were a little tingy but the mids compensated a lot. I wouldn't worry about the rear speakers at all, if you wanna save on cash as you won't really hear them, unless you really want your rear passengers to have decent sound.