Ok sometimes auto parts are imported into Canada and have some minor work done on them. They are immediately sent back across the border after the work is done. Many factors go into determining if the part can be classified as a temporary import. Certain Criteria must be met.
When they do they fall under hs code 9993.00.
This classification allows the parts to be considered tax exempt.
Not all parts crossing will meet the requirements. Hence the automakers being pissed as it will effect their bottom line.