The hard Brexiters' belief that the German car industry will make the EU bend to their will has always been one of their central tenets, as you described about a year ago.

Actually, they believed that already before the 2016 referendum, even Cameron, as argued here.

The main lesson is that they've been constantly misreading Germany, and still do: I saw a bit of BoJo on TV essentially promising that Germany would blink on the brink. Germany's message has always been consistent and is not going to change at the last minute.

German industry cannot save Britain from hard Brexit, warns Merkel  - Torygraph - October 2016

German car makers and other major industrial lobbies will not be able to insist that Britain gets an easy deal in the forthcoming Brexit negotiations, the German chancellor Angela Merkel has warned.

In a further hardening of the line against Britain, Mrs Merkel told the annual conference of German industrial federations, the BDI, that defending the principle of free movement and the internal cohesion of the European Union would come before defending German exports to the UK.

"If we don't say full access to the internal market is linked to full freedom of movement, then a movement will spread in Europe where everyone just does whatever they want," she told business leaders in Berlin.