Actually communicating with one vehicle is the same as the next, because they all use the OBDII protocol. The SAE have the OBDII standard for purchase. While some of the more advanced functions like reprogramming are manufacturer specific the diagnostic items like speed and RPM are the same on every vehicle and outlined in their document(s).

I had the oppritunity to read through the OBDII standard while working on another project, needless to say its tedious to communicate on the buss. I ended up integrating an OBDII to serial IC into my design to simplify things, ELM electronics sells it (ELM327).

The following will give you a taste of the protocol and how it works. This document is geared to VPW but the concepts remain the same.

http://www.lightner.net/lightner/bruce/Lightner-183.pdf