I’ve found this paper [1] which has data from 1960 to 1990 in a plot on page 3. The peak is in 1978 but the number range is very similar to today’s numbers (~1 million employees). I’m assuming the numbers in that chart are in manufacturing alone (didn’t read the paper), because today’s numbers including retail trade, wholesale trade and services add up to more than 4 million jobs, according to my second link above.
Interesting - and fascinating that the oil shock of 73 turned out to be just a blip.
I think it leaves the picture unclear without the associated parts trade, which aren't normally picked out when I've seen news pieces on the auto trade here in Europe. So we get stories and graphs of producing more cars than ever despite the industry employing few. Also leaves me wondering what those staff are doing when car factories themselves have lost such a huge proportion of their workforce.
[1] https://pdfs.semanticscholar.org/24ba/b100ea6eb7d96c9c4247eb...