Ask your local mechanic shops if they can get a factory stereo wiring diagram for you. They should have a program that has all that info on it. Might cost you a little bit of money ($5-$10), but it will save you a butt-load of time.
what type of system are you putting in the sidekick? I have a friend that we put a new Pioneer HU in, with some new 6x9's and cut out the sheet metal underneath the back seat and made some custom boxes for 2 10's. It was a nice system because he still got to use his back seat! If you dont mind, could you send some pics of the install when its done?