Thank you, everyone, for the help. I finally got the harness removed from the head unit yesterday (with only slight breakage) and now I see how the whole thing works. I'll see if I can explain it so that it may be of use to someone else in the future. Basically, the plastic slider slides along four posts, two on each side, of the connection. These posts are part of the head unit plug (also on the harness adaptor if you're using one of those) and the slider part is connected to the harness. When the slider is pushed all the way up, a channel for each of the posts is open so that the two parts can be separated. When the slider is down, it blocks the channel and closes it. So the slider part must be pushed all the way up, and then the harness is pulled away from the head unit. Hopefully this will make sense to the next person that tries to remove their stock head unit for the first time and will be of some help.
Thanks again to everyone for their help. Hopefully I'll finish up my installation tonight and can listen to the good stuff after that.
-garret
If you ever discover that what you're seeing is a play within a play, just slow down, take a deep breath, and hold on for the ride of your life.
-Deep Thoughts by Jack Handey