Canada is not the problem for the US. Our share of vehicle production (13%) is slightly larger than our share of the the number of cars purchased.  Mexico's share of production is closer to 25%, well above the number of cars they purchase.

The average car assembled in Canada has loads of US parts content. The industries are so closely integrated that it would be crazy to apply huge taxes and cause disruption to the industry.

Canada actually runs an auto trade deficit with the US. We import more part + finishes cars than we export.   A tax isn't needed to achieve some measure of equity.