Yes you pretty much said exactly what is required. Based on the NAFTA (North Amerian Free Trade Agreement) any goods or material manufactured within North America is to be free trade between the boarders (meaning no duties). This was done by our governments so they(USA) could capitalize on all of our lumber coming from the north (this benefited the USA more then us, but most things our government does is more for the USA or to follow there suit.)
So short answer yes if is American/Canadian made the you can bring it over the boarders with no hassles.
The other option for car parts is throw out all of your packaging and reciepts and say its used parts you were trying to sell
but this is what you have left. They will save you through because there's no documents or packaging to say where the part originated or the manufactured date.