Audio Systems: 1999 lhs replace factory HU and amp, factory wiring harness, factory amps

Question
Hi Ernesto, I would like to use the factory wiring harness but remove the factory amp (9 speaker Infinity Spatial Imaging System cd/cassette) and add a 4 channel amp to power new front door component spkrs and new rear shelf 3-way spkrs. Also, I would use the new 4-chnnl amp's pre-outs to run a mono-amp/subwoofer. QUESTIONS: Do I need two 4gauge power leads from the battery to the amps? Do I need an heavy duty Optima sealed battery? Can I, in the trunk, tap into the factory amp's wiring harness and use the existing speaker and, also, ant/remote leads for the new amp?  AND, what is the factory amps secondary wiring harness w/10amp fuse with 4 wires: bk/nt (thick gauge) bk/pk (thick) lb/yl (thin) and db/wt (thin) Lastly, since I will be using the 4channel preouts of the new HU to send signal to the new amp, Can I use the new HU's speaker leads,  to power a pair of 3 1/2" speakers I would install somewhere in the shifter console, or in the dash perhaps.  Thanks so much for taking the time to respond. I would love to do this project myself and not have it done for me.

Answer
The infinity amplifier is a lot louder with a new head unit, i keep my infinity amplifier in my Chrysler 300m and add a subwoofer and amplifier for the rear only,
but if you like you can remove the amplifier and all the speakers and use the factory wiring
early 99 comes with the amplifier behind the speaker easy to remove  and you can run all the speaker wires from the new amplifier to the radio location,late 99 to 2004 the amplifier is in the right side of the glove compartment near the dash very difficult to remove that is one of the reasons why i didn't remove, if you remove it all your speaker wires will be there,
4 gauge wire will be good no need for extra battery  3.2 and 3.5 comes with a heavy alternator

good luck