Just went through this and also have the six speakers, factory amp, cd changer..etc. I changed out the head unit with a double din pioneer and integrated the factory amp. It might take a little more then basic audio skills, but I found it extremely easy.
Ill post pictures in this thread.
There are four pre-amp outputs that I used from the rear of this unit and they send sound to the door subs and the front/rear speakers. Changes to fader front to rear work as expected and the subs thump.
REMINDER: in this car, yellow is constant power and Red is switched...get that wrong and you lose all your setting every time you remove the key, gotta pull out unit and swap em.
Car Audio Innovations website is helpful and if you want to buy a kit, their prices on the HU stuff are competitive. Harness stuff is fair. Consider it at least. I just used the advice to make sure I didn't make mistakes.
https://cai-store.com/