Quote: Originally Posted by
20mg 
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)
(emphasis mine)
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.