You can get a build sheet and VIN/RPO information on your car for about $50:
http://www.gmmediaarchive.com/#
Available info:
Buick from 1982
Cadillac from 1980
Chevrolet from 1977
GMC from 1976
Oldsmobile from 1977
Pontiac from 1987
Saturn from 1994
http://www.gmmediaarchive.com/#
Available info:
Buick from 1982
Cadillac from 1980
Chevrolet from 1977
GMC from 1976
Oldsmobile from 1977
Pontiac from 1987
Saturn from 1994