All other things equal, german cars are going to cost as much in China as they do in the USA because China gets no discount because of it's weak currency, in fact when buying abroad it works against them. They still need to pay the germans just as much hard cash as americans do.
The advantage PPP gives poorer countries is that goods that depend on domestic labour, which is very cheap, get to be equally cheap. Any goods or services that don't depend on that cheap pool of domestic labour don't get that benefit. German labour in Germany isn't cheaper to buy in China than it is to buy in the USA, so PPP is irrelevant.
As an aside, German cars are actualy vastly more expensive (up to 2x or 3x for high end models) to buy in China than in most countries due to severe import taxes, but the base cost of buying the car from Germany to ship to China would be about the same.
[Re. Some BMWs made in China - in which case they're not really German cars]