If your car didn't come with the 9 speaker factory bose stereo then by default there will be no speakers for the rear deck and no opening. You can give these 2 speaker adapters a shot.
Metra 82-3005 that will fit in the 6x9 open but can fit a 6'' or 6-3/4'' speaker.
Metra 82-3004 that will fit in the 6x9 opening and fix a regular 6x9 speaker.
As for tapping into the factory amplifier its behind the trunk liner on the right side of the trunk. The amplifier is a silver color. Although its best to run a separate amp to power extra speakers. Lastly you will either need to cut or purchase a used rear deck for speaker grills.