I would like to find out since '96 (1st year of the OBDll terminal) how many cars and light trucks have sold in the US for each of the years from '96 to '08.
Have you tried using a search engine? I would probably start with “US auto sales statistics” as the search phrase.
I’m sure there are probably more direct or targeted sources, but you might take a quick look at the “Statistical Abstract of the US” put out by the Census Bureau. You can find full text pdf here:
http://www.census.gov/compendia/statab/
It contains mountains of stat info for most anything you can think of. Even if it doesn’t get you exactly what you need, all tables provide references to original sources. I’m sure that there will be auto sales/use info in there, and if the stat abstract doesn’t contain the figures you need, just check out the footnotes and you can chase down much more info.