If you can read german this might help you:
http://www.obd-2.de/

Everything I ever needed was there....

elmelectronics have a cheap odb-2 to rs323 chip and some docs:
http://www.elmelectronics.com/Misc/OBDInfo.htm

and there is a scan tool at:
http://freediag.sourceforge.net/


This should be more than enough to get you started.

Actually it's quite easy.

Now if the empeg guys could finally finish that visual-plugin-interface