get a 4 channel amp. chain the front 2 speakers together. and the rear have thare own channel's and thare should be a level nob on the amp so you can turn that down so you dont go deff
so if your front need more power. turn up thare level higher then the rear..
b) wire the computer --> aftermarket amp --> stock amp, and use all 6 speakers (the 200W amp will replace the headunit's 18W amp, hopefully nothing will blow up)
if you can figure out what is the inputs and outputs on ur stock amp u wont even need the aftermarket one. if you arent planning on using a deck and are gonna rune just the car pute. then its possible to take the deck out get the wires that come out of the deck. the green purple white and grey are all to the amp so if you connect those to rca by splicing its possible. may take a few hours but its cheap. just remember on a rca the wire that runs in the middle is positive so run the wires with the black line to them. if u need a more detailed plan id be happy to help.
OP was right with C. Get an line out converter so your stock amp thinks its getting input from a headunit.
You don't want to amplify an amplifier. You'll fry the stock one for sure.
If you want to simply replace the stock amp then you'll need to provide the proper wattage to the stock speakers with an aftermarket amp, otherwise you'll blow the stock speakers. (You'll probably want to upgrade/replace the speakers anyway over time) This may require several separate amps or one big amp to be done properly.
Might be worth saying what car it is, and what model the OE amp is.
To see if anyone has got any info/hack/workaround for it.