I don't understand this and would like a clarification.
There are zero Canadian automobile assemblers; they are Canadian subsidiaries of American companies, with some parts manufacturers being here.
Assuming American companies (and companies from subordinates like Japan) leave, what would be the problem in having a similar relationship with China? Is it because their manufacturing is more integrated?
The American, and other auto makers, only manufacturers in Canada because the Canadian and American auto markets have been integrated for over 80 years. Removing Canada's access to the American market basically makes Canadian auto manufacturing uncompetitive since Canada isn't a big enough market to sustain a domestic auto market.