Ok, here are some US Customs publications on Duties and Tariffs:
Determining Duty Rates
http://www.cbp.gov/xp/cgov/import/du...etermining.xml
Harmonized Tariff Schedule
http://www.usitc.gov/tata/hts/bychapter/index.htm
(Chapter 87 is specifically for motor vehicles; although I'm not sure if this includes aftermarket parts)
General Import Duties
http://www.cbp.gov/xp/cgov/import/duty_rates/
FAQ for importing vehicles or parts
http://help.cbp.gov/cgi-bin/customs....auto+transport
Enjoy!