I have installed my PPI 4 channel amp and Eclipse Head Unit. I had a custom dash plate made and I am extremely happy with the results.
I also swapped the front separates for a set of Boston 5.25's. My OEM front separates are made by Blaupunkt, not Nokia, and they are identical left and right. (I have a 99.5 non bose car).
My question regards bass. I listened to a Bose car, and am pleased with the bass it produces, and like the idea of taking up no trunkspace (my days of dual 12's are over :( ), and not cutting up my deck, and particularly keeping costs down. How does the bose system produce the bass? My understanding is that there are 2 5.25 mid bass drivers in the deck; can I simply buy some Polk Audio midbass drivers, bolt them up to the deck, and get similar results? Can I fit 6.5's back there?

If I do this, should I run them full range, or low pass (my xover is a non adjustable 100hz).

Finally, do I have to install speakers in the rear doors, or would this ruin my front stage?

Thank you in advance for your advice.

In my opinion of bose systems i find them to lack bass, and be midbass heavy.... if u have had 12's then u know that 6.5's will never play the same way...knowing this u have several options. Several company's make 6.5 or 5.25 inch midbass drivers that should fit in the rear deck with minimal work. I would believe a 6.5 could be made to fit without major mod's although i havent done it. Focal makes a 5" SUBwoofer that does actually produce alarmingly low bass... but they cost a couple of hundred bucks EACH. Another option is a powered bass tube... cheap and effective.
I would highly advise avoiding the rear door locations for the reason you state, your front image will move off the dash to over the center console... you bothered to install seperates to help your imaging, dont ruin it. If you want or need anymore info you can email me rather than ramble this thread on.