If you havent already, you might want to get this special harness that allows you to run your aftermarket stereo will still havin your oem plugged in. This will allow you to retain the interior light's ability to turn on when you open your doors. Also, if you got keyless entry you will need it in order to maintain that as well.

You will have to tuck the oem unit somewhere behind your dash - which i was wondering about the other day... Can you somehow eliminate the stereo part of the OEM radio, and just keep the security part? obviously separating the two while maintaining the functionality? My under dash is way cramped. I just ordered a little cubby hole storage piece from Majestic Honda that will mount near the cupholder (where i previously had a tape player) but i was kinda concerned if it would fit with the old oem honda stereo behind there.