If you're willing to do a little wiring, most aftermarket head units should fit just fine in the Insight. Just head to an electronics store like Best Buy and pick up a wiring harness adapter for the car and wire that to the harness that came with whichever new head unit you chose. Most car audio places should be able to do this installation for you if you want. I currently have a JVC Kameleon in my Insight which I installed over 2 years ago, its never had any issues and sounds great. My car originally came with the basic tape deck.

To do an radio install you will have to remove the plastic shroud on the shift tunnel and then remove a few screws on the plastic panel surrounding the radio. The panel will then pull free with a little work. Be careful pulling the panel off because the clips can snap, it is best to work from one corner first and then progressively unsnap each clip one at a time. Once you have uncliped the panel carefully unclip the power cord for the lighter.