"I know that power and access to the speaker system are available from that location." In 1992, I bought a Civic without a radio and added my own. The wiring was already in the doors for the speakers and under the dash for the radio. It sounds like you expect that same sort of pre-installed wiring for the CD changer space in the left trunk area. It's not there.
Apparently, you can add a wiring harness to give the CD changer power and access to the radio/cassette player. The one specifically made for the Insight is quite expensive, though at least one person here has successfully done it with a harness designed for a Civic that cost less (the harness, not the Civic). You have to run that harness through the car from the radio across the dash, down behind the carpet below the dash, under the scuff plate on the door, back through the area below the rear deck... It's a noteworthy project. You can definitely do it, but you'll probably explore your vocabulary of short, emphatic words before you are done.
That harness does not give you direct access to the speakers. You get that through the radio/cassette deck. I hope this helps.