I played with reading GM pre-OBDII data from the ALDL ('assembly line diagnostic link' or something like that -- the plug under the steering column) back when I had a Buick. I built a small circuit which did level conversion for the serial data, and found some links for parsing the data. I got bored with the project before I did much with it, but I still have some links around. Here's one:

http://www.mindspring.com/~amattei/aldl.htm


Now that I have an Empeg, I'm more interested in this kind of thing, since almost all the hardware is already there. But now I have a pre-OBDII Subaru and don't have a source for the data format. (Presumably, getting the car's data stream into the Empeg would be easy; reverse-engineering the meaning of the bits would be a huge project.)