When I was cleaning out my closet this weekend I found the stock radio for my old 2004 S2000 (AP2).
I'm not sure why I upgraded the stereo in the roadster but I happened to keep the factory head unit. It looked like a much better version of the Insight radio (and it had a CD player!!) so I thought I'd see if it would work in the Insight.
Long story short... it was a direct plug in affair

I literally had the stereo in and working in 10 minutes (it's remarkably easy to remove the radio on these cars). The harness in the back plugged right in and all the pins were correct. It took me awhile to find the code for the anti-theft feature on the S2k radio but after putting the code in it worked! It had even saved my old radio stations I was listening to in 2004
So if anyone is looking to upgrade their radio this is a good, cheap, and EASY option that anyone can install