Labour makes up about 15-20% of the cost of a vehicle. Curiously, that number doesn't change all that much between jurisdictions.

And while ~18% is as lot, materials makes up most of the rest, and those costs don't change with jurisdiction. So the OEMs relocate to save a few percent, but mostly they relocate because the overall supply chain is more cost effective. This is why China (and now Vietnam, and Thailand, and before China, Japan and South Korea) are able to do what they do: the government and industry are willing to think long-term and make huge investments to make it happen: slapping down power plants and steel mills and making trade deals with, eg, Africa or the middle east to secure resources at scale.

You're falling for the modern version of blaming the working class--even in a roundabout way--for the capitalists' failure to plan.

Companies seek out cheaper labour, sure, but you're taking a very simplistic view of it:

Canada, the US and Western Europe is a big reason we farm stuff out to cheaper places (like Mexico and China) that don’t have pesky things like high safety standards or employee benefits.

This isn't nearly the case any more, and hasn't been since the 1970s. They actually do have roughly similar safety standards. Replacing workers is expensive, and churn costs a lot, and you really do want to run a plant as efficiently as possible, which means not burning people out. We're not in the triangle shirtwaist era any more.

Workers don't really have much of an impact on the cost or quality of the product because it's cheaper to engineer your plant such that they don't. Mistakes are expensive. Waste is expensive. Re-work is expensive.

If you had said environmental standards, yes, you'd be right. Those can be more lax. That's something different, and also not nearly the gap it used to be.

the fact the huge disparity in labour costs between the two countries is reason the TFW program even works

Slinging donuts at Tim Hortons, answering support calls and/or writing shitty front-end web code is a different thing entirely, and yes the TFW program is a problem, but that's not the issue with heavy industry.