This car is def. not designed for any speaker upgrade at all. I have to take the car to audio excellence, because there is plastic behind the rear speaker, and it is def. way too hot out here in florida to work on a car outside.
In response to yours, and for anybody that is interested in rear speakers. . .
The speaker holes are much to small and shallow to accomodate serious (read serious as "not crap") rear speakers. This presents a conundrum if you want to upgrade fromt he Honda supplied-system.
Much easier than mucking around with building fiberglass housings, worrying about plastic pieces, (unfounded) fears about messing with something behind the bulkhead, why not 'build out' from the holes/wires ya got?
I had a professional installer look at the rear speaker installs for my '00 w/AC. Of course, fiberglass custom enclosures were discussed, but (1) who will ever see them? and (2) fibergalss deadens the liveliness of the sound, plus is difficult to fabricate with. The simplest/best option was to simply make two rings, out of wood (we used fibreboard because of the sound-neutral qualities it has) that act as risers to 'lift' the mount 1.5 inches. The rings look like two oversized washers. This alleviates the problem with mounting deep-profile speakers quite neatly.
Our initial plan was to "wrap" the outside (visible) edge of the painted rings with plastic finishing material so that it would look 100% stock. After sanding and paininting, the rings (painted back) matched the mounted black speakers so well we dispensed with the plastic.
I was lucky the rear speakers had the wiring kit pre-installed.
This upgrade placed 6.5" two-ways in the rear, to complement the 6.5"-speakers alrady mounted in front (in the doors). I am also running a 600w mono sub amp into a Sony Explod 8" sub flush-mounted in the rear. While the car was apart for the sub/rear amp install, 30 pounds of Dynamat was applied to the "trunk" and rear deck. Acoustical foam was applied to "dead spaces located throuought the rear deck. The soft "trunk" was discarded, as that area (above the spare) is occupied with subwoofer enclosure and sound deadening.
I recommend doing the simplest possible procedure. First, the insight is so loud (I have changed the tires from stock to deal witht his) that any system must be designed with the fact in mind that it will never be an 'audiophile system'. Basically you are looking for a system that can overcome the road noise in the car. Second, the rear deck speakers really do not add much to the sound experience. At best, they can be expected to "lift" the soundstage off the floor of the car. A better investment is a class-D amplifier and a small subwoofer.
I can also note that in addition tot he above, I am running a 1-farad capacitor to keep things under control. No problems so far witht eh insight's electrical system runnig this mighty load. Also I have numerous other mods, including a hardwired radar detector, with no electrical complications.