I've always understood it to be Oshawa ON for the 60's/ early 70's cars.  I can't speak to the ones that looked like Chevette's.

73SC wrote:

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Actually we have 68's and up being built in Willow Run Michigan, this has been a topic on here many times before I recall and documentations has been posted to verify this.

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 I looked at two 70 Acadians today.....built in Willow Run......and was kinda shocked.....to see they were NOT assembled in Canada...

  So...were ALL 68-70's built in the US?

Carl Stevenson wrote:

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I believe so, and I think 71 as well, maybe 72.

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 so they still made them after 70?

Carl Stevenson wrote:

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Yes, 71 is the last year for Acadian, and I think it's also the first year for Ventura.

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 That's right an overlap model year. The Pontiac was originally branded Ventura II, playing off the US Ventura full sized car name.