John Wayland, of EV and ShoeBox Subwoofer fame, offers this advice regarding rear speakers. This is from the Yahoo! Honda-Hybrid discussion digest #685 (message 2).

The topic of rear speakers in the Insight has been thoroughly tossed about, year after year, and I've written about it many times. I strongly advise against doing this! The only folks who will tell you how the sound is improved, are those who have never heard an Insight 'properly' set up with just three speakers...two better quality door speakers and a sub (and of course, a 4 ch. amp to run it all with clean, clear power).

Though the Insight indeed, comes with an awful sounding stereo with cheepo door speakers and a lo fi head unit, what some often do not realize, is that the Insight 'does have' near perfectly placed door speaker locations (and horribly placed rear locations). I'm talking about what we car audio competitors call the 'front sound stage'...the one in the Insight is near perfection, just waiting to be unleashed. However, installing 'any' speakers in the behind-the-seats location, absolutely and instantly, ruins this otherwise excellent front sound stage, by pulling the stage to behind the driver and passenger, blurring the front 'imaging', and adding obnoxious hi midbass in the form of unwanted 120-160hz 'boomyness'. Unless you are playing movies through a DVD player in your Insight, there is no reason to actually 'want' to hear sound coming from behind you, and if it's music instead of movie ambiance through the 5.1 audio effect, there's strong reasons against hearing any of the music come from behind the listeners.

Think about it....you are in a darkened hole-in-the-wall type night club, listening to a hot funk band...where are they playing? They're on a stage, in front of you, not behind you! You're at a rock concert in a big stadium...where's the stage and where are you sitting to get the best possible sound? You're at an orchestral event taking in Bach...are you behind the orchestra, or are they in front of you? You're in a recording studio laying down the mix for a band....where are the studio monitors? (hint, I've spent a good deal of time in recording studios, trust me, the monitors are ahead of you, on either side, equidistant from each ear). I could go on, but the point is, especially in the cozy and private environment of a car, you do not want to spoil the excitement and realism that a great front sound stage provides, with rear speakers.