If you replace the HU, then the two door speakers will not work without additional wiring. I suggest replacing the HU and bypassing the factory amp, then installing a component system in the kickpanel and factory speaker location. If you would like the door speakers to play, you could rewire those to be your rear channels and have all the speakers up front working.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>