My local library, via the internet, has a Autotech or something like that where you can get to the wiring diagrams of the vehicles on line. You might give that a try and look around the area you were working on the diagram. They often tell you WHAT it is.
We also have a lot of pull and save type wrecking yards around us, I have found it worth the $3.00 or whatever to be able to see inside the dash on some car that is all ripped up and figure out what things are where they go.
Also you should go over to diymobileaudio (google that) and post up there, just like our christams and decoration community, they are stereo enthusiasts that do it themselves.