Forced labour in Chinese prisons isn't limited to Xinjiang, nor to the car industry. A lot products we use in Europe and North America and elsewhere around the globe are made by Chinese prisoners forced to work under catastrophic conditions.
There is strong evidence for this provided by many independent sources, among them a documentary by Arte (a French-German media outlet). If interested:
A desperate cry for help written in Chinese was discovered in a pregnancy test sold in France and made in a Chinese factory. It revealed a hidden world of Chinese prison-companies where prisoners are forced to work for 15 hour days manufacturing products for export. This documentary tries to find out who wrote the letter.
(And, yes, prison labour exists also in the U.S., and it is as evil, but this doesn't make the autocratic Chinese government any better.)